An Evaluation of the Gregorian and Julian Calendars

Aydın Tiryaki (2025)

^{YouTube EN}

PART 1: PROMPT

“TOPIC 1: THE CONQUEST OF İSTANBUL
The date of the conquest of İstanbul, May 29, 1453, is recorded according to which calendar system? Julian or Gregorian?
What is the Hijri calendar equivalent of this date?
What day of the week does this date fall on?
What sources does this date come from? Ottoman or Byzantine?

TOPIC 2: JULIAN AND GREGORIAN CALENDARS
Can you briefly explain the Julian and Gregorian calendars?
What is the Proleptic Gregorian Calendar?
What are the equivalents of some dates in the Julian Calendar in the Proleptic Gregorian Calendar? January 1st, 4713 BC / January 1st, 45 BC / January 1st, 1 AD / March 21st, 325 AD / May 29th, 1453 AD / October 4th, 1582 AD. What are the characteristics of these days and which days of the week do they fall on?
What is the Julian Day Number (JDN)? According to this system, what is the JDN number of November 26, 2025 in Gregorian calendar terms?

I think it would be more useful if you create paragraphs instead of tables when giving your answers.”

(I wrote the last sentence because it was difficult to transfer the information from the tables into writing.)

CHAPTER 2: EXPERIMENT DESCRIPTION

I conducted an experiment by querying this prompt across 10 different AI models, and in this article, I’ll list the results. I compiled these lists simply to highlight the differences, without commenting on whether any of the information is accurate (aside from minor tweaks).

You can find the details of the responses from the AI ​​models in the appendices.

Gemini: (Appendix 1)
ChatGPT: (Appendix 2)
Grok: (Appendix 3)
Claude: (Appendix 4)
Copilot: (Appendix 5)
Perplexity: (Appendix 6)
Qwen: (Appendix 7)
Kimi: (Appendix 8)
Deepseek: (Appendix 9)
Meta: (Appendix 10)

Before preparing this experiment, I had a long conversation with Gemini about calendar history and calendar systems. At the end of this conversation, I asked Gemini for a summary. Because this summary can be an informative resource, I am publishing it in the appendix (Appendix 11).

CHAPTER 3: THE CONQUEST OF İSTANBUL

Question: Which calendar system corresponds to the conquest of İstanbul, May 29, 1453? Julian or Gregorian?

Answer : All models reported that May 29, 1453 was the date according to the Julian calendar.

Question: What is the Hijri calendar equivalent of this date?

Answer:
Gemini: The 20th day of the month of Jumada al-Akhir (Jumada al-Akhir) in the year 857
ChatGPT: 20 Jumada al-Akhir 857
Grok: 29 Muharram 857 (some sources say 27-30 Muharram)
Claude: 27 Rajab 857 or 21 Muharram 858
Copilot: 20 Jumada al-Awwal 857
Perplexity: 25 Shawwal 857
Qwen: 20 Jumada al-Awwal 857
Kimi: Rabi al-Awwal 857 or Rabi al-Akhir 857
Deepseek: The 20th day of the month of Shaban in the year 857
Meta: 20 Jumada al-Akhir 857
The answers are very diverse and this diversity also stems from the differences in expression of the hijri calendar, but it is also understood that some of them are very different.

Question: What day of the week does this date fall on?

Answer:
Gemini: Tuesday
ChatGPT: Tuesday
Grok: Tuesday
Claude: Monday
Copilot: Tuesday
Perplexity: Monday (In another previous query, said Wednesday)
Qwen: Tuesday
Kimi: Tuesday
Deepseek: Tuesday
Meta: Tuesday
There are 3 different answers. The most common answer is ‘Tuesday’ (8/10).

Question: What day in the Proleptic Gregorian calendar corresponds to the Julian date May 29, 1453?

Answer:
Gemini: June 7, 1453
ChatGPT: June 7, 1453
Grok: June 7, 1453
Claude: June 8, 1453 Copilot: June 7, 1453
Perplexity: June 8, 1453
Qwen: June 7, 1453
Kimi: June 2, 1453
Deepseek: June 7, 1453
Meta: May 29, 1453
There are 4 types of answers. The most answered one was June 7, 1453 (6/10).

CHAPTER 4: JULIAN-GREGORIAN EQUIVALENTS

What are the equivalents of some dates in the Julian calendar in the Proleptic Gregorian calendar? What are the characteristics of these dates, and which days of the week do they fall on?

Question: What is the equivalent of January 1, 4713 BC (Julian) in the Proleptic Gregorian Calendar? What day of the week does it fall on?

Answer: The Julian Day Number (JDN) is the day the counting begins. This date is a reference point used in astronomical calculations.

Gemini: Monday, November 24, 4714 BC
ChatGPT: Monday, November 24, 4714 BC
Grok: Monday, November 24, 4714 BC
Claude: Monday, January 1, 4713 BC
Copilot: Monday, November 24, 4714 BC
Perplexity: Monday, January 1, 4713 BC;
Qwen: Monday, January 1, 4713 BC
Kimi: Monday, November 24, 4714 BC
Deepseek:- Monday
Meta: Monday, January 1, 4713 BC
There are 2 different answers. The most common answer is ‘Monday, November 24, 4714 BC’ (5/10)

Question: What is the equivalent of January 1, 45 BC (Julian) in the Proleptic Gregorian calendar? What day of the week does it fall on?

Answer: It is the first day of the Julian Calendar’s official entry into force.

Gemini: Monday, January 1, 45 BC
ChatGPT: Saturday, December 30, 46 BC
Grok: Monday, January 2, 45 BC
Claude: Monday, January 1, 45 BC
Copilot: Wednesday, December 30, 46 BC
Perplexity: Monday, January 1, 45 BC
Qwen: Sunday, January 1, 45 BC
Kimi: Wednesday, January 1, 45 BC
Deepseek: Friday, October 2, 46 BC
Meta: Saturday, January 2, 45 BC

There are 4 different answers. There is disagreement about the correct date.

Question: What is the equivalent of January 1, 1 AD (Julian) in the Proleptic Gregorian calendar? What day of the week does it fall on?

Answer: This is the date accepted as the beginning of the Gregorian era.

Gemini: Sunday, January 3rd, 1 (Changed it to December 30th 1 at the first warning.)
ChatGPT: Monday, January 3rd, 1 (After a long discussion, we agreed on December 30th 1.)
Grok: Saturday, January 3rd, 1
Claude: Saturday, January 1st, 1
Copilot: Saturday, January 3rd, 1
Perplexity: Monday, January 1st, 1
Qwen: Monday, January 3rd, 1
Kimi: Monday, January 1st, 1
Deepseek: Saturday, January 1st, 1
Meta: Saturday, January 3rd, 1

Although there were two types of answers, when Gemini and ChatGPT were asked again, they were given the information ’30 December 1 BC’, which is not on this list and is likely to be correct.

Question: What is the equivalent of March 21, 325 AD (Julian) in the Proleptic Gregorian calendar? What day of the week does it fall on?

Answer: This date is the Spring Equinox of the year in which the Council of Nicaea was held. The Council fixed the equinox as March 21 for determining the date of Easter.

Gemini: Tuesday, March 20, 325
ChatGPT: Monday, March 20, 325
Grok: Monday, March 24, 325
Claude: Monday, April 8, 325
Copilot: Sunday, March 20, 325 (approximately)
Perplexity: Monday, March 23, 325
Qwen: Sunday, March 23, 325
Kimi: Sunday, March 21, 325
Deepseek: Sunday, March 20, 325
Meta: Thursday, March 21, 325

There are 5 types of answers. The most common answer was ‘March 20, 325’ (4/10).

Question: What is the equivalent of October 4, 1582 AD (Julian) in the Proleptic Gregorian calendar? What day of the week does it fall on?

Answer: This date is the last Julian day before the official implementation of the Gregorian calendar. The following day, according to the Julian calendar, should have been October 5, 1582, but with the Gregorian reform, the calendar was moved forward by 10 days to October 15, 1582.

Gemini: Thursday, October 14, 1582 (First said October 4, corrected accordingly)
ChatGPT: Thursday, October 11, 1582
Grok: Thursday, October 14, 1582
Claude: Wednesday, October 4, 1582
Copilot: Thursday , October 14, 1582
Perplexity: Friday, October 15, 1582
Qwen : Thursday, October 14, 1582
Kimi: Thursday, October 4, 1582 
Deepseek: Thursday, October 14, 1582 
Meta: Thursday, October 14, 1582

This information is common because it’s accessible without requiring calculation. Many hesitate to provide this information directly, preferring to use the phrase “October 15, 1582, is the following day.”

Question: What is the Julian Day Number (JDN)? According to this system, what is the JDN number of the Gregorian date November 26, 2025?

Answer: The Julian Day Number (JDN) is a sequential number system used in astronomy that expresses the number of complete days that have elapsed since a specific starting point. The epoch was determined as Greenwich Noon (12:00 UT) on January 1, 4713 BC (November 24, 4714 BC according to the Proleptic Gregorian calendar), and JDN 0 was assigned to this moment. This system allows dates from different calendar systems (Julian, Gregorian, Hijri, etc.) to be expressed with a single, continuous, and simple sequence of numbers and is indispensable for astronomical calculations.

Gemini: 2,460,996
ChatGPT: 2,461,006
Grok: 2,461,027
Claude: 2,461,100 (approximately)
Copilot: 2,461,342
Perplexity: 2,460,303
Qwen: 2,460,994
Kimi: 2,460,621
Deepseek: 2,460,619
Meta: 2,461,011

This is a calculable number, and 10 answers came up, but 10 different ones. No comment!

CHAPTER 5: NEW PROMPT

I thought there should be a new prompt (promo) and prepared the following question:

Which date was the Gregorian Calendar Reform undertaken on October 4, 1582 (Julian) to fix?
a) January 1, 45 BC
b) January 1, 1 AD
c) March 21, 325 AD
What would be the equivalent of the date in your answer in the Proleptic Gregorian Calendar?
If the Julian and Proleptic Gregorian dates are different in your answer, could you answer the following question: “If the goal was to fix that date, shouldn’t the Julian and Proleptic Gregorian dates have remained the same on that date?”

CHAPTER 6: RESPONSES TO NEW PROMPTS

Question: The Gregorian Calendar Reform, which took place on October 4, 1582 (Julian), was carried out to fix which date?

Answer: All 10 AI models that answered the question responded 325 on March 21. This was the expected answer because it was recorded information.

Question : What would be the equivalent of the date in your answer in the Proleptic Gregorian Calendar?

Response:

Gemini: March 21, 325
ChatGPT: March 23, 325
Grok: March 18, 325
Claude: March 18, 325 and March 21, 325 (somehow both dates are in the same reply)
Copilot: March 20, 325
Perplexity: March 21, 325
Qwen: March 18, 325
Kimi: March 20, 325
Deepseek: March 20, 325
Meta: March 21, 325

There were four different answers to this question.

Question: If the Julian and Proleptic Gregorian dates are different in your answer, could you answer the following question: “If the goal is to fix that date, then shouldn’t the Julian and Proleptic Gregorian dates have remained the same on that date?”

Response:

Gemini: Yes.
ChatGPT: No.
Grok: No.
Claude: Yes or No.
Copilot: No.
Perplexity: Yes.
Qwen: No.
Kimi: No.
Deepseek: No.
Meta: Yes.

There were two different answers and one that was unclear. Is the majority answer, “No,” the correct answer?

CHAPTER 7: NO COMMENTS!

I conducted an experiment where I received multiple answers to many questions. For some models, when I didn’t get a complete answer the first time, I asked again to fill in the gaps, and sometimes the answers were different from the answers given a few minutes earlier.

There were times when I was not convinced about the answer I received, especially when I insisted that the well-known models were wrong, and they gave the right answer.

Due to inconsistencies in the answers (I don’t mean incorrect), I created a new prompt and received different answers to the questions in that prompt. I want to emphasize that because the prompt text is guiding, I suspect some of the answers may have changed.

I leave the results of this experiment to the readers of the comments by saying “No Comment!”

Aydın Tiryaki
Ankara, November 26, 2025

INFORMATION NOTES

1. Free Plan opportunities were used in all artificial intelligence models except Gemini and CoPilot.
2. When I published this article, I provided the AI ​​models that could create visuals with a link to the article and a prompt asking, “Based on this content, can you create an image that best represents what’s being discussed here?” The results were added to each model’s section. While most of the models created visuals, some also prepared prompts for the visuals.
3. The information in the appendices of this article is included in the text by copy/paste from 10 different pages, so there are different formats.

APPENDIX 1: GEMINI

Great, let’s take a closer look at these important historical and calendar systems!

Conquest of İstanbul (May 29, 1453)

The date of the conquest of İstanbul, May 29, 1453 , is according to the Julian Calendar, which was widely used in both Ottoman and Byzantine lands at the time .

Calendar and Day Information

• Calendar System: May 29, 1453 is a date given according to the Julian Calendar .
• Hijri Calendar Equivalent: This date corresponds to the 20th day of the month of Jumada al-Ahir (Jumada al-Ahir) of the year 857 in the Hijri calendar.
• Day of the Week: According to the Julian calendar, May 29, 1453, falls on a Tuesday . The conquest is documented using this Julian calendar system, which was also used by Byzantium at the time, and its corresponding Tuesday day.
• Source: This date is a record from the Julian calendar used in Byzantine and Ottoman sources at the time of the conquest . The conquest was a shared event that influenced the historical record of both civilizations.

Julian and Gregorian Calendars

Julian Calendar

The Julian calendar was introduced by Julius Caesar in 45 BC to correct irregularities in the Roman calendar at that time. This calendar was based on the solar year and used a year in days. To compensate for this excess, he introduced a leap year (366 days) every four years (years divisible by 4 without a remainder). The Julian calendar served as the standard calendar system in Europe and the regions surrounding the Mediterranean for centuries. However, a Julian year is approximately minutes longer than a true tropical year (approximately 10 days). This small difference accumulated over time, and by the 16th century, the calendar’s seasonal difference (especially around the vernal equinox) had exceeded 10 days.

Gregorian Calendar

The Gregorian calendar was promulgated and enacted by Pope Gregory XIII in 1582 to correct this accumulated error in the Julian calendar, and particularly to correct the date of Easter . This calendar is also based on the solar year, but it incorporates more precise leap year rules. In the Gregorian calendar , a year is a leap year if it is evenly divisible by 4; BUT , if it is evenly divisible by 100, it is not a leap year; BUT, if it is still evenly divisible by 400, it is still a leap year. So, 1700, 1800, and 1900 are not leap years, while 1600 and 2000 are. This revised rule makes the calendar more accurate by bringing the average year length much closer to the true tropical year.

Proleptic Gregorian Calendar

The Proleptic Gregorian Calendar is a hypothetical system created by applying the rules of the Gregorian calendar to dates prior to October 4, 1582 , the date the calendar was officially adopted . This calendar is used by historians and astronomers to compare events before 1582 consistently with the current Gregorian system. For example, it is used to calculate the Gregorian day on which an event in 1453 fell, even though the Gregorian calendar did not yet exist at that time. The 10-day difference between the Julian and Gregorian calendars applies to 1582, but this difference decreases as we go back, reaching approximately 2 days by the 1st century BC. All calculations in this system are made backwards, based on the current Gregorian calendar rules.

Correspondences of Important Dates

The following dates are significant turning points in astronomy, calendar systems, or historical events. The corresponding dates in parentheses are the days on which the date falls according to the Proleptic Gregorian Calendar :

• January 1, 4713 BC (Gregorian: November 24, 4714 BC): This day marks the beginning of the Julian Day Number (JDN) counting and falls on a Monday of the week. This date is a reference point used in astronomical calculations.
• January 1, 45 BC (Gregorian: January 1, 45 BC): This date is the official entry into force and the first day of the Julian Calendar . This day falls on a Monday of the week . The Proleptic Gregorian equivalent is the same because the difference between the two calendars had not yet been established at this time.
• January 1, 1 AD (Gregorian: January 3, 1 AD): According to the Proleptic Gregorian system, January 1, 1 AD in the Julian calendar corresponds to January 3, 1 AD (a difference of 2 days). This date falls on a Sunday of the week . This is the date considered the beginning of the Gregorian era.
• March 21, 325 AD (Gregorian: March 20, 325 AD): This date is the Spring Equinox of the year in which the Council of Nicaea was held . The Council fixed the equinox as March 21 for the determination of the date of Easter. This day falls on a Tuesday of the week . In the Proleptic Gregorian calendar, it falls on March 20, with a difference of one day.
• May 29, 1453 AD (Gregorian: June 7, 1453 AD): The date of the conquest of İstanbul in the Julian calendar. This date falls on a Tuesday . In the Proleptic Gregorian calendar, this date falls on a Wednesday , a difference of nine days from June 7, 1453 .
• AD 4 October 1582 (Gregorian: AD 4 October 1582): This date was the last Julian day before the official implementation of the Gregorian calendar . It falls on a Thursday of the week. The following day, which should have been October 5, 1582 according to the Julian calendar , was advanced 10 days to October 15, 1582 , following the Gregorian reform.

Julian Day Number (JDN)

The Julian Day Number (JDN) is a sequential number system used in astronomy that expresses the number of complete days that have elapsed since a specific starting point. The epoch was determined as Greenwich Noon (12:00 UT) on January 1, 4713 BC (November 24, 4714 BC according to the Proleptic Gregorian calendar) , and JDN 0 was assigned to this moment. This system allows dates from different calendar systems (Julian, Gregorian, Hijri, etc.) to be expressed with a single, continuous, and simple sequence of numbers and is indispensable for astronomical calculations.

• Gregorian JDN Number of November 26, 2025: The Julian Day Number of November 26, 2025 at noon (12:00 UT) according to the Gregorian calendar is 2,460,996 .

I hope you found this detailed historical and calendar information helpful.

In light of this information, perhaps you would like to learn about the formulas for converting from the Hijri calendar to the Gregorian calendar ?

GEMINI RESPONSE TO THE NEW PROMPT:

The Gregorian Calendar Reform was undertaken to fix the date of 21 March 325 AD (option c).

This date is the date on which the First Council of Nicaea fixed the vernal equinox (equal day and night) for the calculation of Easter . However, due to accumulated errors in the Julian calendar, by 1582 the vernal equinox had actually moved back by 10 days. The Gregorian reform aimed to correct this 10-day shift and restore the equinox to March 21.

Equivalent in the Proleptic Gregorian Calendar

The Proleptic Gregorian Calendar equivalent for the date AD 21, 325 , which the Gregorian Calendar Reform aimed to fix , is AD 21, 325 .

Why did they remain the same? The reform of 1582 AD aimed to restore the equinox date (March 21) established by the Council of Nicaea to its correct place. The Proleptic Gregorian Calendar is achieved by applying the rules of the Gregorian calendar (specifically, the leap year rule) backward to dates before October 4, 1582.

The year 325 AD is not a leap year (not divisible by 400) in the Proleptic Gregorian Calendar, nor is it a leap year (divisible by 4) in the Julian calendar. However, at this early date, the difference between the Julian and Proleptic Gregorian calendars did not yet exceed 1 day. By 325 (when the Julian calendar was further advanced), the difference between the two calendars was approximately 1 day .

However, especially when considering a key reference point such as the fixation of Easter , historians and astronomers generally accept March 21st of AD 325 as March 21st in both calendars because that was the decision of the Council of Nicaea and the basis for the reform. Calendar differences began to accumulate over the centuries preceding the reform.

Should Julian and Proleptic Gregorian Dates Remain the Same?

“If the goal is to fix that date, then shouldn’t the Julian and Proleptic Gregorian dates remain the same on that date?”

The answer to this question is yes, it is expected to remain the same for this purpose and is considered the same in the context in which the reform is made .

Description:

1. Fixing the Goal: The aim of the reform was to fix the calendar date of the spring equinox (March 21) in 325 AD . By 1582, March 21 in the Julian calendar was 10 days after the true astronomical spring equinox .
2. Reference Point: The year 325 AD was adopted as the reference point for calculating Easter due to a decision of the Council of Nicaea . This reference point, ignoring theoretical differences between calendars, adopted March 21 as the starting point.
3. Effect of the Reform: The Gregorian reform removed the intervening 10 days from the calendar to move the astronomical position of 325 to 1582. This brought the spring equinox back to March 21 in 1582.
4. Proleptic Application: The Proleptic Gregorian calendar is simply a Proleptic application of the rule. However, even this application may differ slightly from the Julian calendar, particularly in the very early periods up to 4 AD (because the leap year rule was not yet established). Despite this, March 21 in 325 AD, the date on which the reform was based , is generally accepted as March 21 in both calendars as the reference point .

In short, March 21, 325 AD , is the starting point and reference point for the reform. Even if there is a theoretical day difference between the calendars, this date is fixed by the spirit and purpose of the reform, and its Proleptic Gregorian equivalent is also taken as March 21, 325 .

APPENDIX 2: ChatGPT

Below, I have summarized both topics in paragraphs , clearly and accurately, and the technical transformations of the dates.

TOPIC 1: CONQUEST OF İSTANBUL

Which calendar system is May 29, 1453?

The date Mehmed the Conqueror conquered İstanbul was May 29, 1453 (Tuesday), a date given according to the Julian calendar
. At that time, all of Europe and Byzantium were using the Julian calendar. The Ottoman Empire, however, was not yet officially using the Rumi calendar ; its historical records used the Hijri lunar calendar. Therefore, the term “May 29” is the Julian date derived from Byzantine/European chronicles , and modern historians use it as such.

In the proleptic (Proleptic) Gregorian calendar, this date would be June 7, 1453 .

What is the Hijri calendar equivalent of this date?

29 May 1453 Julian → 20 Jumada al-Akhir 857. In
Ottoman chronicles, the conquest is generally recorded with this Hijri date.

What day of the week does this date fall on?

• Julian 29 May 1453 → Tuesday
• Proleptic Gregorian equivalent of June 7, 1453 → Tuesday

(That is, there is no shift in the day between the two calendars; only the date has shifted.)

What sources does this date come from?

• The expression “May 29” comes from records made by Byzantine and Latin sources according to the Julian calendar.
• Ottoman sources give the day of the conquest in Hijri .
• In modern historiography, the Julian date used by the Byzantine side is generally taken as is.

TOPIC 2: THE JULIAN AND GREGORIAN CALENDARS

Julian Calendar

The Julian calendar was implemented by Julius Caesar in 45 BC.
Each year has 365 days , and every fourth year has a leap year (366 days). Because it adds
approximately 11 minutes per year , the seasons and the calendar drift over the centuries.

Gregorian Calendar

It was enacted by Pope Gregory XIII in 1582.
To correct for seasonal shift:

• Years divisible by 100 are not leap years (1700, 1800, 1900…),
• However, those that are divisible by 400 are leap years (1600, 2000).

It is the calendar used in the world today.

What is the Proleptic Gregorian Calendar?

The Gregorian calendar did not exist before 1582, but in modern historical research, this system is used to answer the question, “What date would past events have occurred if the Gregorian calendar had been used?”
This is the Gregorian calendar imaginarily extended into the past .

Julian ↔ Proleptic Gregorian Equivalents of Important Dates and Days of the Week

The following explanations are summary:

January 1, 4713 BC (Julian Day = 0)

• This date marks the beginning of the Julian Day Number (JDN) system used in astronomy.
• The Proleptic Gregorian equivalent is November 24, 4714 BC .
• The day of the week is calculated as Monday .

January 1, 45 BC (beginning of the Julian calendar)

• Proleptic Gregorian equivalent December 30, 46 BC
• The day of the week is Saturday .

January 1, 1 AD

• There is a difference of 2 days between Julian and Proleptic Gregorian.
• Julian: January 1st → Saturday
• Gregorian: January 3, 1 → Monday

March 21, 325 AD (Nicea Council definition of Easter)

• Julian: March 21
• Proleptic Gregorian: 20 March 325
• Day of the week: Monday

May 29, 1453 AD (Conquest of İstanbul)

• Julian: May 29, 1453 → Tuesday
• Proleptic Gregorian: June 7, 1453 → Tuesday

October 4, 1582 AD

• Last day of the Julian calendar (Italy, Spain, Portugal).
• The next day: October 15, 1582 Gregorian
• Julian: Thursday , October 4, 1582

Julian Day Number (JDN)

The JDN is a continuous day count dating back to 4713 BC .
It is used in astronomy, calendar conversions, and chronology. It changes
daily at 12:00 UT .

Gregorian JDN value of November 26, 2025

With the direct conversion algorithm:

JDN = 2461006

(This value is a verified mathematical calculation.)

If you want, I can also calculate day of the week, JDN or Hijri – Gregorian conversions for other dates.

ChatGPT RESPONSE TO NEW PROMPT:

Let me explain step by step and with full technical accuracy :

1. What date was the Gregorian Reform made to fix?

Correct option: c) 21 March 325 AD

FROM WHERE?

• The Council of Nicaea (325) accepted the solstice (spring equinox) as 21 March 325 when calculating the date of Easter .
• However, by the 1500s the equinox had shifted back 11 days due to the annual drift in the Julian calendar .
• The reform was undertaken to re-fix the vernal equinox to March 21 .

The aim of the reform was therefore to re-establish the astronomical equinox date of March 21, 325, in its correct place in the calendar.

2. What is the Proleptic Gregorian equivalent of this date (March 21, 325)?

Stage:

The Julian → Proleptic Gregorian difference of 325:

• In 325 the Julian–Gregorian difference was ≈ 2 days (1.94 days exactly, but 2 days when rounded to calendar days).

Because:

**March 21, 325 (Julian)

= 23 March 325 (Proleptic Gregorian)**

3. Then a legitimate question arises:

“If the goal is to fix the date of March 21, 325, why isn’t this date the same in the Julian and Proleptic Gregorian calendars at that time?”

This is a very important point, and one that many people misunderstand.
The answer is:

The reform was not intended to equalize the calendar with the old dates—it was intended to move the equinox back to March 21 in 1582.

Gregorian reform:

• He did not aim to “correct” past mistakes day by day,
• He simply wanted the date of 1582 to be brought back into astronomically correct position .

Therefore:

• March 21, 325 was accepted as astronomically “March 21”,
• But the backwards version of the Gregorian calendar (proleptic Gregorian) is a theoretical calculation tool and has not been used historically.

So the reform did not:

“Let’s make March 21st of the year 325 the same day in Julian and Gregorian.”

The Reformation did the following:

“The equinox shifted in 1582, let’s push it back and reduce its shift from now on.”

4. So the question is:

“If the goal was to use March 21, 325 as a reference, why weren’t the Julian and Proleptic Gregorian dates made the same?”

Because the aim of the reform was not to achieve calendar parity with old dates, but to stop future drift.

• According to Julian, past events have already happened.
• Since the Gregorian calendar did not exist before 1582 ,
  • In ancient times, Julian and Proleptic Gregorian are naturally different.

This difference is not an error , but is due to the mathematical “post-application” of the proleptic Gregorian to the past.

Brief Summary

Question	Reply
What date did the Reformation fix?	21 March 325 (Equinox of the Council of Nicaea)
The Proleptic Gregorian equivalent of this date?	March 23, 325
Why are the two calendars not the same?	Because the reform was to correct the equinox of 1582 , not to ensure calendrical equality in 325.

APPENDIX 3: GROK

TOPIC 1: CONQUEST OF İSTANBUL

The date used for the conquest of İstanbul is May 29, 1453, according to the Julian calendar . The Gregorian calendar was not yet in use in 1453 (the Gregorian calendar came into effect in 1582). Therefore, both Ottoman and Byzantine sources cite the conquest as May 29, 1453, according to the Julian calendar.

The Hijri calendar equivalent of this date is 29 Muharram 857 (some sources show slight differences between 27 and 30 Muharram, the most widely accepted is 29 Muharram).

May 29, 1453, falls on a Tuesday in the Julian calendar . Both contemporary Byzantine sources (Doukas, Kritovoulos, Sphrantzes) and Ottoman chronicles (Tursun Bey, Aşıkpaşazade) clearly state that this day was Tuesday. Mehmed the Conqueror’s entry into the city also occurred on a Tuesday.

The date is essentially the same in both Ottoman and Byzantine sources. The most definitive contemporary sources are Doukas and Kritovoulos on the Byzantine side and Tursun Bey on the Ottoman side; all give the same Julian date.

TOPIC 2: THE JULIAN AND GREGORIAN CALENDARS

The Julian calendar was a system instituted by Julius Caesar in 45 BC that assumed an average year length of 365.25 days and added a leap year (February 29) every four years. However, because the true tropical year was 365.2422 days, the Julian calendar advanced approximately 1 day every 128 years.

The Gregorian calendar is the version corrected by Pope Gregory XIII in 1582. The leap year rule was changed: years divisible by 100 are not considered leap years, but those divisible by 400 are still leap years (for example, 1700, 1800, and 1900 are not leap years, while 2000 is). Additionally, in 1582, the calendar was moved forward by 10 days (October 15 was declared the day after October 5). This makes the Gregorian calendar only 26 seconds per year, or about 1 day in 3,300 years.

The Proleptic Gregorian Calendar means that the Gregorian rules are applied backwards without interruption. This means that we calculate dates before 1582 according to Gregorian leap year rules. This system is used by historians and astronomers to represent ancient dates consistently with the current calendar.

Below are the Julian and Proleptic Gregorian equivalents of the dates you want and the days of the week (calculations are based on astronomical standards):

• January 1, 4713 BC (Julian) → Proleptic Gregorian: November 24, 4714 BC → Monday (This date is the beginning of the Julian Day Number system, JDN 0 starts at noon)
• January 1, 45 BC (Julian) → Proleptic Gregorian: January 2, 45 BC → Monday (considered the first day of the Julian calendar)
• January 1 AD (Julian) → Proleptic Gregorian: January 3 AD → Saturday (Beginning of “Anno Domini” in some traditions)
• March 21, 325 AD (Julian) → Proleptic Gregorian: March 24, 325 → Monday (Opening date of the 1st Council of Nicaea)
• May 29, 1453 AD (Julian) → Proleptic Gregorian: June 7, 1453 → Thursday (So the conquest actually took place on Thursday, June 7, 1453, according to today’s calendar)
• AD 4 October 1582 (Julian) → The next day became Gregorian directly as October 15, 1582 → Thursday → Friday (the day the Gregorian reform began; October 5–14, 1582 never occurred)

The Julian Day Number (JDN) is a continuous day counting system used in astronomy. JDN 0 begins at noon (UTC) on January 1, 4713 BC, in the Proleptic Gregorian calendar. It is a whole day number only (the fractional part is used for noon).

The Julian Day Number of the Gregorian date of November 26, 2025 is JDN 2461027. (Calculation: JD 2461027.5 starts at noon, the full day is JDN 2461027.)

I hope this paragraph-style explanation was what you wanted!

GROK RESPONSE TO NEW PROMPT

Correct Answer: c) 21 March 325 AD

Explanation: The sole and primary purpose of Gregory XIII’s Gregorian Calendar Reform of 1582 was to ensure that the vernal equinox (spring equinox) remained fixed on March 21. This date was the equinox date adopted by the First Council (Council of Nicaea, 325 AD) as the reference date for calculating Easter.

• Because the leap year rule in the Julian calendar was too simple (a leap year every 4 years), the calendar gradually drifted away from the true solar year.
• By 1582 the true spring equinox had now fallen around March 11 .
• The reform not only skipped 10 days (Thursday, October 4, 1582, was followed by Friday, October 15, 1582) but also corrected the leap year rule.
• Purpose: To re-establish and permanently fix the date of the equinox on March 21, as accepted by the council in 325 AD .

So the date the reform wanted to fix was 21 March 325 (the date of the Council of Nicaea and the equinox on that date).

Let’s eliminate the other options:

• a) January 1, 45 BC → It is the beginning of the Julian calendar, it has nothing to do with the reform.
• b) January 1 AD → the beginning of the Christian calendar (Anno Domini), but not the target of the reform.

The equivalent of the date in question in the Proleptic Gregorian Calendar

March 21, 325 (Julian) = March 18, 325 (Proleptic Gregorian)

(At the time of the Council of Nicaea, the Julian calendar was in use. According to the Proleptic Gregorian calculation, the difference was about 3 days at that time.)

Answer to NEW Question:

“If the goal is to fix that date, shouldn’t the Julian and Proleptic Gregorian dates remain the same on that date?”

No, it wasn’t necessary and it wasn’t expected to remain the same.

Reason:

• The Gregorian reform called for the future equinox to remain on March 21.
• The real equinox date in the past (in the year 325) was already accepted as March 21 in the Julian calendar (so the council decided).
• But in 1280, the Julian calendar had drifted by about 10 days. The Gregorian system both corrected this drift and prevented future drift.
• The proleptic Gregorian calendar is merely a mathematical extension. The 1582 reform was not undertaken to rewrite the past ; it was simply to ensure that March 21st would remain the equinox day in the future .
• So, in 325, while it was March 21st in Julian, it was March 18th in Proleptic Gregorian, which is completely normal and does not contradict the purpose of the reform.

In summary:

• Wanted to be fixed: March 21 (equinox day)
• Reference year: 325 (Council of Nicaea)
• The actual (Julian) date at that time: March 21, 325
• Proleptic Gregorian equivalent: March 18, 325
• The difference is not a failure of the reform; on the contrary, it shows the accumulated error of the Julian calendar in the past and proves that the Gregorian system corrected this error.

APPENDIX 4: CLAUDE

A Comprehensive Study of the Conquest of İstanbul and Calendar Systems

TOPIC 1: CONQUEST OF İSTANBUL (29 MAY 1453)

Which calendar system indicates the date of the conquest?

The date of the conquest of İstanbul, May 29, 1453, is according to the Julian calendar . Because this date was recorded by Ottoman sources and the Gregorian calendar was not yet officially used in the Muslim world at the time, it was expressed using the Julian system. The Gregorian calendar was only officially declared in 1582 by Pope Gregory XIII. Therefore, although there are date conversions made by Western sources, the primary historical source remains the Julian calendar.

Conquest Date Hijri Calendar Equivalent

The date of the conquest of İstanbul is recorded as either Rajab 27, 857 , or Muharram 21, 858 , according to the Hijri calendar. According to Ottoman sources, the conquest occurred on a Monday, Rajab 27, 857. Because the Hijri calendar was the official calendar system in the Muslim world, this date is a more consistent and reliable source for Islamic history. When calculating between the Hijri and Gregorian calendars, the Hijri year 857 corresponds to the Gregorian calendar of 1453-1454.

What day of the week does the conquest date fall on?

May 29, 1453 (Julian calendar) falls on a Monday . Ottoman sources also record that “the conquest took place on Monday.” If calculated retroactively using the Gregorian calendar, the same day corresponds to June 8, 1453, which remains a Monday. The algorithms used to calculate the day of the week yield the same results in both calendar systems because the days of the week are continuous.

Sources of Conquest History: A Comparison of Ottoman and Byzantine Sources

The most comprehensive and reliable information on the conquest of İstanbul is provided by Ottoman sources. These sources include Kritovolos’s translation from the Greek original, Tursun Bey’s History, Şükrullah’s Behcetü’t-Tevârîh, İdris Bitlisî’s Heşt Behişt (Eight Heavens), and Mustafa Ali’s Künhü’l-Ahbâr. These sources consistently date the conquest as 27 Rajab 857 AH (29 May 1453 Julian).

Byzantine sources, however, are limited and were mostly written after the city’s fall. Late Byzantine historians such as Doukas, Choniates, and Laonikos Chalkokondyles mentioned the date of the conquest, but there are some inconsistencies. In particular, some Byzantine sources in Western languages ​​record the conquest as May 22, 1453, or other dates. However, when comparing Ottoman and Byzantine sources, May 29, 1453, is the most widely accepted date. Byzantine sources adopt a more emotional and narrative approach to the conquest of İstanbul, while Ottoman sources provide more systematic, historically and textually based information.

TOPIC 2: THE JULIAN AND GREGORIEN CALENDARS

Julian Calendar

The Julian calendar was established in 45 BC by Emperor Julius Caesar on the advice of the Egyptian astronomer Sosigenes. This calendar system is based on the principle that a year is approximately 365.25 days long. In the Julian calendar, a leap year is implemented every four years, meaning there are years with 366 days. This brings the calendar closer to the solar (tropical) year. The number of months and days in the Julian calendar is similar to the system we use today: January 31, February 28 or 29, and March 31. However, a historical problem arose: the Julian calendar’s calculations are slightly longer than the actual solar year (365.2425 days). This difference accumulates by about one day every 128 years. Over the course of sixteen centuries, this difference has persisted for several days; as a result, the spring equinox (March 21) has gradually shifted towards May. Easter calculations for church calendars have become inaccurate.

Gregorian Calendar

The Gregorian calendar was created by Pope Gregory XIII in 1582 as a reform. The primary purpose of the Gregorian calendar was to correct deviations from the Julian calendar. In the Gregorian system, a leap year is observed every four years; however, centennial years (such as 1700, 1800, and 1900) are determined by special rules. A centennial year is considered a leap year only if it is divisible by 400 without a remainder. For example, 1600 and 2000 are leap years, but 1700, 1800, and 1900 are not. Thanks to this rule, the Gregorian calendar more closely matches the true solar year (365.2425 days). The error rate is approximately one day in 3000 years. The Gregorian calendar was first adopted in the Papal territories, then spread to Catholic countries, then to other European countries, and finally to the entire world. The Gregorian system is the official calendar used today.

Proleptic Gregorian Calendar

The Proleptic Gregorian Calendar consists of a mathematical extension of the rules of the Gregorian calendar into the past, including periods before the calendar system was historically implemented. In other words, although the official implementation of the Gregorian calendar began in 1582, the system is theoretically applied backwards, including to dates before Christ. This method is particularly used in disciplines such as science and history to translate ancient dates into the modern calendar system. For example, a date recorded in the Julian calendar can be translated using the Proleptic Gregorian Calendar. This allows events historically recorded in different calendar systems to be converted to a common reference system (the Gregorian) and compared.

Historical Dates and Gregorian Equivalents

January 1, 4713 BC (4713 BC) : In the Julian calendar, this date falls on a Friday. In the Proleptic Gregorian calendar, the same date corresponds to a Monday . This date is an important starting point for astronomical calculations and is considered the zero point of the Julian Day Number (JDN) system. The difference between them is four days. This difference is due to the divergence between the Julian and Gregorian calendar systems. This difference increases with increasing date.

January 1, 45 BC (45 BC) : This year marks the official implementation of the Julian calendar. In the Julian calendar, this date falls on a Saturday. In the Proleptic Gregorian calendar, it corresponds to a Monday . The difference between them is two days. This date marks the beginning of the Julian calendar reform, but astronomically it is related to the calculations of Sosigenes of Alexandria.

January 1, 1 AD (January 1, 1 AD) : This date falls on a Monday in the Julian calendar. It corresponds to a Saturday in the Proleptic Gregorian calendar . The difference between them is two days. This date is considered the starting point (epoch) of the Christian calendar system.

March 21, 325 AD (March 21, 325 AD) : This date is significant as the date of the First Council of Nicaea, which made decisions regarding the calculation of Easter. In the Julian calendar, this date falls on a Wednesday. In the Proleptic Gregorian calendar, it corresponds to a Monday . The difference between them is two days. The council used the spring equinox (March 21) as the basis for calculating Easter; however, over time, this date shifted in the Julian calendar.

May 29, 1453 AD (May 29, 1453 AD) : As mentioned, this day, the date of the conquest of İstanbul, falls on a Monday in the Julian calendar. In the Proleptic Gregorian calendar, it corresponds to Saturday, June 8, 1453. The difference between them is 10 days. This difference demonstrates the extent of the divergence between the Julian and Gregorian calendar systems by 1453. Had the calendar reform not been implemented in 1582, this difference would have increased over time.

October 4, 1582 AD (October 4, 1582 AD) : This date is known as the final Julian date of the calendar reform implemented by Pope Gregory XIII. In the Julian calendar, this date falls on a Wednesday. In the Gregorian calendar, it began on Friday, October 15, 1582, with 11 intervening days (October 5-14). This date marks the final point at which the Julian and Gregorian calendar systems diverge.

Julian Day Number (JDN)

The Julian Day Number (JDN) is a date-independent day counting system used for astronomical and computational purposes. This system uses Monday, January 1, 4713 BC, as the epoch and assigns a consecutive number to each day. The JDN system allows for accurate calculations between dates in different calendar systems. For example, if the difference between a date in the Julian calendar and a date in the Gregorian calendar needs to be calculated, the JDN is calculated for both dates and their difference is calculated. This system is particularly widely used in astronomical observations, satellite calculations, and scientific research.

The formulas used to calculate JDN use month, day, and year information. Special correction coefficients are applied when calculating the JDN for a date in the Gregorian calendar. Let’s calculate the Julian Day Number for November 26, 2025 : November 26, 2025 in the Gregorian calendar corresponds to a number around 2460640 or 2460641 in terms of JDN. Scientific formulas such as the Meeus algorithm are used for the exact calculation. The formula is as follows: If months are 1 and 2, decrease the year by 1 and add 12 months; a = (14-month)/12; y = year + 4800 – a; m = month + 12 a – 3; JDN = day + (153 m + 2)/5 + 365*y + y/4 – y/100 + y/400 – 32045. According to this formula, the JDN number for November 26, 2025, is around 2461100 (the exact value must be verified when calculated using software). This system allows scientists and historians to evaluate dates and their relationships within a universal system.

CLAUDE’S RESPONSE TO THE NEW PROMPT

The main purpose of the Gregorian Calendar Reform was to fix the date of 21 March 325 AD . The correct answer is c) 21 March 325 AD.

This date was established as the reference point for the calculation of Easter, as adopted by the Council of Nicaea. Due to the deviation from the Julian calendar, the calculation of Easter had become increasingly inaccurate. The Gregorian Reform was undertaken to correct this reference date and ensure that Easter was always calculated on the Sunday after the first full moon after March 21 AD.

Equivalent in the Proleptic Gregorian Calendar:

The Julian date of March 21, 325 AD would be March 18, 325 AD in the Proleptic Gregorian Calendar . (Since the Gregorian Calendar did not yet exist at the time, applying it retroactively would result in a difference of 3 days.)

Answer to Your Question:

Yes, that’s a very logical observation! If the goal is to fix March 21, 325 AD, then in theory that date should remain the same in both the Julian and Proleptic Gregorian calendars. But the distinction here is important:

• Julian 21 March 325 = the actual date of the historical event
• Gregorian March 21, 325 = a hypothetical date used only for retrospective calculation

The purpose of the reform was to coordinate future dates from 1582 onward, not to rewrite past history. Your historical record remains Julian; after 1582, it remains Gregorian. Therefore, there is no conflict—the two calendar systems were used in different eras and are calculated retroactively for comparison today.

APPENDIX 5: COPILOT

You’ve asked some very good and comprehensive questions. Let me explain them in paragraphs:

1. Conquest of İstanbul (May 29, 1453)

May 29, 1453 , the accepted date of the conquest of İstanbul, is according to the Julian calendar . This is because the Gregorian calendar was not adopted by Pope Gregory XIII until 1582 ; ​​therefore, the calendar used in Europe and the Byzantine world in 1453 was the Julian. Converted to Gregorian, this date becomes June 8, 1453 .

The Hijri calendar equivalent of this date is 20 Jumada al-Awwal 857. In Ottoman sources, the conquest is recorded with this Hijri date.

In terms of the day of the week, May 29, 1453 falls on a Tuesday in the Julian calendar.

As for the source, the date of the conquest is recorded by both Ottoman chroniclers (e.g., Tursun Bey, Aşıkpaşazade) and Byzantine historians (e.g., Dukas, Kritovoulos). However, the “May 29” reference is based on the Byzantine Julian calendar, while the Ottomans wrote it using the Hijri calendar. In other words, the date is fixed as it is today based on the convergence of the two records.

2. Julian and Gregorian Calendars

The Julian calendar was established in Rome by Julius Caesar in 46 BC. It has 365 days per year and a leap day (366 days) every four years. However, the true solar year is 365 days, 5 hours, and 49 minutes; because the Julian calendar is 365 days, 6 hours, it accumulates an error of approximately 1 day every 128 years.

The Gregorian calendar was adopted by Pope Gregory XIII in 1582 to correct this error. The leap year rule is more complex: years divisible by 4 are leap years, but years divisible by 100 are not; only years divisible by 400 are leap years. This results in a system much closer to the solar year.

The Proleptic Gregorian Calendar is an artificial application of the Gregorian calendar to dates before 1582. That is, historians or astronomers use this method when they want to express past events with the Gregorian system.

3. Special Dates and Their Equivalents

• January 1, 4713 BC : The starting point of the Julian Day Number (JDN) system used by astronomers . This date falls on a Monday in the Julian calendar.
• January 1, 45 BC : This is the date the Julian calendar actually came into effect. It falls on a Wednesday.
• January 1, AD 1 : The beginning of the Gregorian year count. It is a Saturday in the Julian calendar.
• March 21, 325 AD : The date on which the Council of Nicaea convened is important for Easter calculations. It falls on a Sunday.
• May 29, 1453 AD : Conquest of İstanbul. It is a Tuesday.
• October 4, 1582 AD : Last day of the Julian calendar; the Gregorian calendar was adopted the following day. It is a Thursday.

These dates are turning points in terms of both calendar reforms and religious/astronomical calculations.

4. Julian Day Number (JDN)

The Julian Day Number (JDN) is a continuous day numbering system used by astronomers. It uses January 1, 4713 BC (Julian) as its starting point , and from that date onward, the days are numbered consecutively. This system simplifies date calculations because it eliminates calendar discrepancies.

The Gregorian date you asked about , November 26, 2025 , corresponds to the number 2461342 in the JDN system . This is used as a unique day ID in astronomers’ calculations.

In summary: The conquest of İstanbul took place on Tuesday, May 29, 1453, on 20 Jumada al-Awwal 857 AH according to the Julian calendar. Converted to the Gregorian calendar, this becomes June 8, 1453. The difference between the Julian and Gregorian calendars stems from their sensitivity to the solar year. The Julian Day Number system is the universal day counting system used to overcome these differences.

If you’d like, I can show you the Gregorian equivalents and JDN numbers of these dates , calculated in more detail. Would that interest you?

Julian dates and their retroactive Gregorian equivalents

• Feature: Date chosen as the start of the Astronomical Julian Day Number (JDN). Julian: January 1, 4713 BC. Proleptic Gregorian: November 24, 4714 BC. Day of the week: Monday.
• Characteristics: Beginning of the period in which Julius Caesar’s calendar reform was implemented; the practical implementation of the Julian calendar. Julian: January 1, 45 BC Proleptic Gregorian: December 30, 46 BC Day of the week: Wednesday
• Feature: “AD” is the date considered the beginning of the year count (reference of the Anno Domini system). Julian: January 1, AD Proleptic Gregorian: January 3, AD Day of the week: Saturday (Julian), Monday (Gregorian)
• Feature: Date known for the Council of Nicaea’s fixation of the equinox as “March 21” for the calculation of Easter (according to the Julian calendar). Julian: March 21, 325 AD Proleptic Gregorian: March 20, 325 AD (approximately) Day of the week: Sunday (Julian)
• Feature: Conquest of İstanbul. Julian: May 29, 1453 AD Proleptic Gregorian: June 7, 1453 AD Day of the week: Tuesday (Julian), Thursday (Gregorian)
• Feature: Last Julian day just before the Gregorian reform (in Rome the following day was skipped to 15 October 1582). Julian: 4 October 1582 AD Gregorian (actual usage after the reform): 14 October 1582 AD (retroactive equivalent) Day of the week: Thursday (in both expressions)

Notes:

• “Proleptic Gregorian” equivalents are mathematical applications of Gregorian rules to dates before 1582; ​​they are not the actual official usage of the period.
• Due to historical deviations in leap year applications in early periods (especially in the centuries BC and early AD), some day-week calculations may differ slightly from source to source; those given here are widely accepted astronomical conversions.
• In the 15th century, the Julian–Gregorian difference was 9 days; so the Julian date of 29 May 1453, when converted to Gregorian, corresponds to 7 June 1453.

COPILOT RESPONSE TO NEW PROMPT

The Goal of the Gregorian Reformation

The Gregorian Calendar Reform was implemented on October 4, 1582 (Julian) . The reform’s purpose was to fix the date of the Vernal Equinox (spring solstice) . This equinox was accepted as March 21 at the Council of Nicaea (325 AD). Therefore, the correct option is: c) March 21, 325 AD

Backward Gregorian Equivalent

Now let’s get to the critical point:

• AD 21 March 325 (Julian) → The Proleptic Gregorian equivalent is March 20, 325. Because by the year 325, the Julian calendar had shifted forward by about 1 day relative to the Gregorian .

The Logical Contradiction You Asked About

“If the goal is to fix that date, then shouldn’t the Julian and Proleptic Gregorian dates remain the same on that date?”

Actually, no. Because the purpose of the reform was not to make a past date exactly equal , but to stop the equinox from shifting in the future .

• At the Council of Nicaea, March 21 (Julian) was the equinox day.
• By 1582 the equinox had shifted to March 11 on the Julian calendar .
• The reform moved the calendar forward by 10 days, bringing the equinox back to around March 21 (Gregorian) .

So the Gregorian reform took the Julian date of March 21, 325 , as its reference point ; but although the Gregorian equivalent of this date was different, the important thing was that the spring equinox fell on March 21 again .

Summary

• Target of the reform: 21 March 325 AD (equinox of the Council of Nicaea)
• Proleptic Gregorian equivalent: March 20, 325
• There is no contradiction, because the aim was not to “equalize the date” but to “fix the equinox”.

APPENDIX 6: PERPLEXITY

The conquest of İstanbul was dated May 29, 1453, according to the Julian calendar then in effect. The Gregorian calendar was not yet in use at the time; this system was implemented in 1582. The Hijri equivalent of this date is Shawwal 25, 857. May 29, 1453, falls on a Monday. This information is found in both Ottoman and Byzantine sources. While Ottoman sources use this date to describe the conquest of Mehmed the Conqueror, Byzantine historians also recorded the same date according to the Julian calendar.

Julian and Gregorian Calendars

The Julian Calendar is a calendar implemented by Julius Caesar in 46 BC, which calculates a year as 365 days and 6 hours. The Gregorian Calendar was introduced in 1582 by Pope Gregory XIII to correct the deviation from the Julian Calendar. A year in the Gregorian Calendar is calculated as 365 days, 5 hours, 49 minutes, and some changes were made to the leap year application to prevent a 3-day difference in 400 years.

Proleptic Gregorian Calendar

The Proleptic Gregorian Calendar is a calendar created by applying the Gregorian system to the period before 1582. This system is used in astronomical calculations. For example, May 29, 1453 in the Julian Calendar corresponds to June 8, 1453 in the Proleptic Gregorian Calendar. The days of the week for these dates also differ. The Gregorian equivalents of some important dates in the Julian Calendar and the day of the week are as follows: January 1, 4713 BC (Julian) – January 1, 4713 BC (Gregorian): Monday; January 1, 45 BC (Julian) – January 1, 45 BC (Gregorian): Monday; January 1, 1 AD (Julian) – January 1, 1 AD (Gregorian): Monday; March 21, 325 AD (Julian) – March 23, 325 AD (Gregorian): Monday; May 29, 1453 AD (Julian) – June 8, 1453 AD (Gregorian): Monday; October 4, 1582 AD (Julian) – October 15, 1582 AD (Gregorian): Friday. These dates are important in astronomical and historical research.

Julian Day Number (JDN)

The Julian Day Number (JDN) is a system used in astronomy and expresses the number of days since January 1, 4713 BC. The Gregorian JDN number of November 26, 2025 is 2460974. This system is used especially in dating astronomical events and very long-term calculations.

This information is based on sources and calendar conversion tools used in historical research.

PERPLEXITY RESPONSE TO NEW PROMPT

The Gregorian Calendar Reform, implemented on October 4, 1582, was specifically aimed at fixing the date of the Spring Equinox on March 21, 325. This date is the equinox day used as a reference in the calculation of Easter by the Council of Nicaea.

Response and Date Correspondence

Correct answer:
c) 21 March 325 AD .​

The equivalent of this date in the Proleptic Gregorian Calendar is again   considered the Gregorian date of March 21, 325 , because the Gregorian Calendar Reform aimed to keep the equinox date fixed at the Council of Nice.

The Question of Date Incompatibility

If the goal was to fix that date, the Julian and Gregorian Proleptic dates should have been the same at that time. However, the Gregorian Calendar Reform shifted the date of the equinox at the Council of Nice by 10 days in 1582 to keep it fixed. This ensures that the date at the Council of Nice remains the same day in both the Julian and Gregorian calendars, because the Gregorian Calendar references the date at the Council of Nice and corrects for the errors of the Julian Calendar, keeping that date fixed.

APPENDIX 7: QWEN

Qwen3-Max

Of course! Your questions are quite detailed and cover historically deep topics. Below, I’ve answered your questions in a sequential and comprehensive manner under two main headings. All information is based on accepted sources on historical science and calendar systems.

TOPIC 1: CONQUEST OF İSTANBUL

Which calendar system does the conquest of İstanbul, May 29, 1453, follow? Julian or Gregorian?
May 29, 1453, is based on the Julian calendar. This is because the Gregorian calendar was introduced by Pope Gregory XIII in 1582. Therefore, neither Western Europe nor the Ottoman Empire used the Gregorian calendar in 1453. While the Ottomans used the Hijri calendar as their official calendar system, the Christian world (and therefore the Byzantine Empire) adopted the Julian calendar. Therefore, the dates for the conquest of İstanbul are given using the Julian calendar in contemporary Christian sources, and the Hijri calendar in Muslim sources.

What is the Hijri equivalent of this date?
The Hijri equivalent of May 29, 1453 (Julian calendar) is calculated as 20 Jumada al-Awwal 857. However, some sources may have minor discrepancies because the Hijri calendar is based on the sighting of the crescent moon, and regional variations and observational errors can cause a few days’ deviation. However, the general consensus is that 857 falls in the spring month of Jumada al-Awwal.

What day of the week does this date fall on?
May 29, 1453, falls on a Tuesday. This has been confirmed by astronomical and calendar conversion calculations based on the Julian calendar.

What sources provide this date? Ottoman or Byzantine?
The date of May 29, 1453, appears in both Ottoman and Byzantine (Christian) sources, but this date belongs to Christian/Byzantine sources. Ottoman sources generally record the events according to the Hijri calendar. For example, the chroniclers of Mehmed the Conqueror use the term “Jumada al-Awwal, year 857” for the day of the conquest. In contrast, Christian writers such as the Byzantine historians Doukas and Leonardo di Tocco place the event as May 29, 1453, according to the Julian calendar. Therefore, the term “May 29, 1453” has generally been quoted in historical literature from both Western and Byzantine sources.

TOPIC 2: THE JULIAN AND GREGORIAN CALENDARS

Could you briefly explain the Julian and Gregorian calendars?
The Julian calendar was implemented in the Roman Empire by Julius Caesar in 45 BC. In this calendar, the year was defined as 365.25 days; that is, February 29th was added every four years to create a leap year. However, because the true solar year was approximately 365.2422 days, the Julian calendar was off by approximately one day every 128 years. This error necessitated correction, particularly because it distorted the timing of Easter in church calendars.

Therefore, the Gregorian calendar was established in 1582 by Pope Gregory XIII, following suggestions from astronomers (notably Aloysius Lilius and Christopher Clavius). The rule of leap years was changed in the Gregorian calendar: years divisible by 100 are not leap years (e.g., 1700, 1800, 1900), but those divisible by 400 are (e.g., 1600, 2000). This correction brought the calendar much closer to the solar year. Today, almost all countries in the world use the Gregorian calendar.

What is the Proleptic Gregorian Calendar?
The Proleptic Gregorian Calendar is an imaginary calendar system derived by applying Gregorian rules to dates before 1582. Because the Gregorian calendar did not exist at that time, this system is used primarily for comparative analysis in astronomy and history. For example, if we want to calculate the Gregorian equivalent of 1 AD, we use the Proleptic Gregorian calendar.

What are the equivalents of some dates in the Julian calendar in the Proleptic Gregorian calendar?
Below, I’ll explain the Julian and Proleptic Gregorian equivalents of the dates you mentioned, their characteristics, and the days of the week:

• January 1, 4713 BC: This date is the starting day of the Julian Day Number (JDN) system (JDN = 0). It is a date in the Julian calendar and also the beginning of the “Julian Period” used in astronomical calculations. This date remains the same as January 1, 4713 BC according to the Proleptic Gregorian calendar, because from that period onward, the difference between the two calendars has not yet occurred. Day of the week: Monday (according to astronomical conventions).
• January 1, 45 BC: The year Julius Caesar introduced the Julian calendar. This date is in the Julian calendar. According to the Proleptic Gregorian calendar, January 1, 45 BC is still considered the same day because the Gregorian difference is still small. Day of the week: Sunday.
• January 1, AD 1: Beginning of the year considered the birth of Jesus (he was actually probably born around AD 4-6). This is according to the Julian calendar. In Proleptic Gregorian, this date translates to January 1, AD 3, because there is a difference of about two accumulated days by then. Day of the week: Saturday (Julian), Monday (Proleptic Gregorian).
• AD 21, March 325: This is the date of the Council of Nicaea, which decided how to calculate Easter. The calendar used at the time was the Julian calendar. According to the Proleptic Gregorian calendar, this date would be approximately March 23, 325. Day of the week: Sunday (Julian).
• May 29, 1453 AD: Date of the conquest of İstanbul. In the Julian calendar. According to the Proleptic Gregorian calendar, this date becomes June 7, 1453, because a difference of approximately nine days had accumulated between the Julian and Gregorian calendars by 1453. Day of the week: Tuesday (the same day in both systems, as the difference is only a matter of date shift; the week cycle remains unchanged).
• AD 4 October 1582: The last day of the Julian calendar. The following day was declared October 15, 1582, by decree of Pope Gregory XIII, meaning that 10 days were skipped between October 5 and 14, 1582. This date officially marked the beginning of the Gregorian calendar. Day of the week: Thursday (Julian), Friday (Gregorian).

What is the Julian Day Number (JDN)? According to this system, what is the JDN number for the Gregorian date November 26, 2025?
The Julian Day Number (JDN) is a continuous day counting system used in astronomy and chronology. In this system, which began on January 1, 4713 BC (Julian calendar, noon), each day is represented by a consecutive integer. This system is the most reliable way to compare different calendar systems and calculate differences between dates. The JDN is independent of time zones and begins at 12:00 noon UTC.

The JDN number for November 26, 2025, according to the Gregorian calendar, is 2460994. This value can be verified against international astronomical standards (such as the US Naval Observatory algorithms or Jean Meeus’ astronomical formulas). Because this date is the “current real date” you specify, the JDN calculation is up-to-date and accurate.

I hope these explanations have answered all your questions clearly and concisely. If you require any further details or additional explanations, I’d be happy to assist.

QWEN ANSWER FOR NEW QUESTION

The Gregorian Calendar Reform , carried out by Pope Gregory XIII on 4 October 1582 (Julian calendar) , was primarily aimed at fixing the date of the Spring Equinox (i.e. 21 March ) determined by the Council of Nicaea (325 AD) .

Correct answer:

c) March 21, 325 AD

The equivalent of this date in the Proleptic Gregorian Calendar is:

• According to the Julian calendar, March 21, 325 ,
• When applied retroactively to the Gregorian calendar, it becomes March 18, 325 .

Why?Because the Julian calendar was slightly longer (~365.25 days) than the actual year (~365.2422 days of the solar year), it gradually became out of sync with the seasons. Between 325 AD and 1582, this deviation accumulated by about 10 days . Therefore, the Gregorian reform of 1582 closed this gap by introducing October 15 after October 4. If the same correction is applied retroactively, March 21 in 325 AD would appear as Julian and March 18 in the Gregorian calendar .

Additional question:

“If the goal is to fix that date, then shouldn’t the Julian and Proleptic Gregorian dates remain the same on that date?”

No, it is not necessary. Because the goal of the reform is to fix astronomical reality (specifically, the Spring Equinox being around March 21st), not calendar dates .

• In 325 AD , with the Julian calendar , March 21 was indeed astronomically the Spring Equinox .
• But by 1582, the small error in the Julian calendar had accumulated, and the equinox had shifted to around March 11 .
• The Gregorian reform was made to ensure that the equinox occurs around March 21st again.
• So, while the astronomical target remains the same , the calendar dates shift. In 325 AD, March 21 in the Julian calendar is retroactively calculated to March 18 in the Gregorian calendar—because the Gregorian calendar uses a more accurate solar year model.

Therefore, “fixing the date” means fixing the astronomical event, not the calendar label . Therefore, it is normal and expected that Julian and Proleptic Gregorian dates are not the same .

Summary:

• Correct option: c) 21 March 325 AD
• Proleptic Gregorian equivalent: March 18, 325
• It is normal that they are not the same , because the goal is to fix the astronomical event (equinox), not the date tag.

APPENDIX 8: KIMI

Let me answer your questions about the conquest of İstanbul and calendar systems in detail:

1. HISTORY OF THE CONQUEST OF İSTANBUL

Calendar System: May 29, 1453, is based on the Julian calendar . The Julian calendar was used throughout the Christian world and Europe until 1582. The Gregorian calendar was not yet in use in 1453, having been officially declared by Pope Gregory XIII in 1582.

Hijri Calendar Equivalent: The Hijri equivalent of this date is Rabi’ al-Awwal 857 or Rabi’ al-Akhir 857 (the year 857). Because the Ottomans used the Hijri calendar during the conquest, the date was recorded this way. The siege began on April 6, 1453 (Julian), and ended 53 days later, on May 29.

Day of the Week: May 29, 1453, falls on a Tuesday according to the Julian calendar . This is a detail mentioned in both Ottoman and Byzantine sources.

Source: The date comes from both Ottoman and Byzantine sources. The most important Byzantine source describing the fall is the chronicle of Giorgios Sfrantzes , advisor to the last emperor . Ottoman sources include works by contemporary historians such as Tursun Bey and Aşikpaşazade . Both sides consistently use the date of May 29, 1453.

2. Julian and Gregorian Calendars

Julian Calendar: The calendar system established by Roman Emperor Julius Caesar in 45 BC. It assumes a year length of 365.25 days and adds a leap day every four years. This system calculates years as either 365 or 366 days (leap years).

Gregorian Calendar: Our current calendar. It was established by Pope Gregory XIII in 1582. It calculates the year length as 365.2425 days and applies more complex leap year rules: years divisible by 4 are leap years, but years divisible by 100 are not, and years divisible by 400 are also leap years.

Proleptic Gregorian Calendar: A system that adapts the Gregorian calendar to dates before 1582. It is used to calculate dates before 1582 according to the Gregorian rule. This is standard practice for historians and astronomers because the Gregorian calendar, developed directly from the Julian calendar, can be extrapolated backwards.

3. EQUIVALENTS OF THE SPECIFIED DATES

January 1, 4713 BC: The starting point of the Julian day number system (JDN = 0). It is a Monday in the Proleptic Gregorian calendar . This date is the reference point for astronomical calculations.

January 1, 45 BC: The year in which Julius Caesar’s calendar reform began. It is a Wednesday in the Gregorian calendar . This is the last year before Caesar’s death.

January 1, AD 1: The administrative date considered the beginning of the Gregorian calendar. It is a Monday in the Gregorian calendar and is used for approximate calculation of the Nativity date.

March 21, 325 AD: This is the date of the Council of Nicaea. It is considered the date of Jesus’ crucifixion. It is a Sunday in the Gregorian calendar and has symbolic significance for the Christian world.

May 29, 1453 AD: The date of the conquest of İstanbul. It is a Tuesday in the Julian calendar . According to Gregorian retrospective calculations, it is also a Tuesday (June 2, 1453).

October 4, 1582 AD: This is the date the Gregorian calendar officially began. Going to bed on a Thursday evening and waking up on Friday morning as October 15, 1582 , 10 days were lost. This was used for preparation and creates a 10-day gap in the middle of the date.

4. JULIAN DAY NUMBER (JDN)

The Julian Day Number (JDN) is a continuous day number system used for astronomical calculations. It numbers each day with a consecutive integer, starting at 12:00 on January 1, 4713 BC (according to the Julian calendar).

This system is used to easily calculate the difference between dates and to date astronomical events.

Gregorian JDN for November 26, 2025: 2460621

Here is the detailed calculation:

• Year: 2025
• Month: 11 (November)
• Day: 26

JDN formula:

Copy

a = (14 – months) / 12

y = year + 4800 – a

m = month + 12a – 3

JDN = day + (153m + 2)/5 + 365y + y/4 – y/100 + y/400 – 32045

According to this formula, the JDN for November 26, 2025 is calculated as 2,460,621 . This is the total number of days since January 1, 4713 BC.

KIMI ANSWERS TO THE NEW PROMPT

Answer: c) 21 March 325 AD

The Gregorian Calendar Reform was carried out on October 4, 1582, to restore the position of the Spring Equinox in the calendar to its position at the time of the 1st Council of Nicaea in 325 AD , by skipping ten days (following October 4 to October 15)..

1. The Council of Nicaea (325) decided to use the vernal equinox as March 21st for calculating the date of Easter . However, the Julian calendar’s calculation of years as 365.25 days (11 minutes and 14 seconds longer than the actual year) resulted in a shift of approximately 10 days in 1257 years. In 1582, the equinox no longer occurred on March 21st, but on March 11th. The Gregorian reform aimed to offset this 10-day difference and reposition the equinox on March 21st.

The date in your answer corresponds to the Proleptic Gregorian Calendar:

March 21, 325 (Julian) = March 20, 325 (Proleptic Gregorian)

Why is it different? The difference between the two calendar systems varies from year to year. By the year 325, the Julian calendar had moved ahead by about one day . Therefore, when we apply the Gregorian rules backward, we get March 20, 325 (Proleptic Gregorian) instead of March 21, 325 (Julian).

If the goal is to fix that date, then shouldn’t the Julian and Proleptic Gregorian dates remain the same at that time?

No, it didn’t have to stay the same , and in fact, it wasn’t. The purpose of the Gregorian reform was not to replicate the errors of the Julian calendar, but to establish a more precise system that accurately reflected astronomical reality .

1. The main difference : The Julian calendar’s 365.25-day year was already inaccurate. Even in 325, it was 11 minutes longer than the actual year. The Gregorian calendar, however, calculated the year as 365.2425 days, which was much closer.

2. Practical goal : The reform aimed not to answer the question, “What was March 21st in 325?” but rather to “ return the astronomical equivalent of the day designated as March 21st in 325 (the vernal equinox) to March 21st.” At that time, March 21st of the Julian calendar was already 2-3 days off the astronomical equinox.
3. Resetting the Accumulation : The fundamental logic of the reform was to clear 1,257 years of accumulated errors and prevent future slippage . This also means correcting past errors.

In short, calendar reform does not imitate an existing faulty system; it corrects and improves it . Therefore, the Julian and Proleptic Gregorian dates of 325 are not, and have never been, necessarily consistent.

APPENDIX 9: DEEPSEEK

That’s a great question. I’ll answer this comprehensive question in paragraph format, as you requested, under two main headings.

TOPIC 1: CONQUEST OF İSTANBUL

The conquest of İstanbul, dated May 29, 1453, is based on the Julian calendar, which was officially used by both the Christian Byzantine Empire and the Muslim Ottoman Empire at the time. The Gregorian calendar was adopted much later, in 1582. Therefore, the conquest date is May 29, 1453, according to the Julian calendar.

While the equivalent of this date in the Hijri calendar may vary by a day depending on calculation methods, the generally accepted date is  the 20th of Sha’ban, 857 Hijri.  This is supported by the fact that fetihnames (letters announcing the conquest) were written with this date in the Islamic world and by the records of prominent Islamic historians of the period.

The day of the week on which May 29, 1453, falls has been precisely determined through retrospective astronomical and calendar calculations. According to these calculations, the glorious day when İstanbul was conquered by Mehmed the Conqueror fell on a  Tuesday  .

Regarding the source of this date, May 29, 1453, is a date commonly used in both Ottoman and Byzantine sources. Since both sides used the Julian calendar at the time, the date given for the conquest is the same in both cultures. Therefore, the date is based on both Ottoman and Byzantine sources. The difference lies in the interpretation and interpretation of the date; for one side, it is the “conquest,” while for the other, it is the “fall of the city.”

TOPIC 2: THE JULIAN AND GREGORIAN CALENDARS

The Julian Calendar was created by Roman Emperor Julius Caesar in 46 BC and came into effect in 45 BC. This calendar uses a year of 365.25 days and compensates for this quarter-day by implementing a leap year every four years. However, because the true solar year (tropical year) is approximately 365.2422 days, the Julian calendar overcalculates by approximately 11 minutes per year. This small error accumulated over the centuries, putting the calendar ahead by approximately one day every 128 years.

The Gregorian Calendar  was adopted by Pope Gregory XIII in 1582 to correct this deviation from the Julian calendar. The correction was made in two ways: 1) 10 days were skipped in 1582 (the day after October 4th was declared as October 15th). 2) The leap year rule was improved. Years divisible by 100 are not considered leap years unless they are also divisible by 400 (for example, 1700, 1800, and 1900 are not leap years, while 1600 and 2000 are). This made the calendar extremely accurate and is today the most widely used calendar in the world.

The Proleptic Gregorian Calendar is the application of the rules of the Gregorian calendar to dates prior to the calendar’s official adoption in 1582. This is an academic method used to provide a standard reference point for historical and astronomical calculations. For example, we can calculate the Julian date of May 29, 1453, for the conquest of İstanbul in the Proleptic Gregorian calendar. Although the 10-day difference had not yet occurred at the time, this date, according to Proleptic Gregorian calculation,   falls on Wednesday , June 7, 1453 .

The Proleptic Gregorian equivalents and characteristics of the important dates you provided are as follows:

• January 1, 4713 BC:  This date is the starting point of the Julian Day System. The Julian Day Number (JDN) for this day is 0. This is considered the universal starting point for counting days.
• January 1, 45 BC:  One of the first days of the Julian calendar. According to the Proleptic Gregorian calendar,  it is October 2, 46 BC.
• January 1, 1 AD:  This date, considered the beginning of the Anno Domini (AD) era, is the same in both calendars. It is considered to fall on a Saturday of the week.
• AD 21 March 325:  This is the date the Council of Nicaea convened, and the spring equinox was fixed as this date for determining the date of Easter. According to the Proleptic Gregorian calendar,  it is March 20, 325 .
• May 29, 1453 AD:  Conquest of İstanbul, as stated above. Tuesday according to Julian, Wednesday (June 7) according to Proleptic Gregorian.
• AD 4 October 1582:  The day the Gregorian calendar reform was implemented. This was a Thursday in the Julian calendar. The following day was declared Friday, October 15, 1582, due to a 10-day correction in the calendar.

The Julian Day Number (JDN) is a continuous day counting system that expresses the number of days elapsed since noon (Greenwich Mean Time) on January 1, 4713 BC. This system is used by astronomers and historians to compare dates between different calendar systems or to easily calculate the difference in days between dates.

Standard formulas or online tools are used to calculate the Gregorian Day Number (JDN) of November 26, 2025. According to this calculation,  the Julian Day Number (JDN) of Wednesday, November 26, 2025 is 2,460,619.

DEEPSEEK RESPONSE TO ADDITIONAL PROMPT

That’s a great and detailed question. Let’s answer it first, then explain the second part.

Answer: c) 21 March 325 AD

One of the primary goals of the Gregorian Calendar Reform was to restore the date of the Spring Equinox to March 21, as established by the Council of Nicaea in 325, following the birth of Jesus  . Due to an error in the Julian calendar, the equinox had shifted by approximately 10 days in 1582 (falling on March 11 instead of March 21). Pope Gregory XIII implemented the reform to correct this shift and restore the equinox to March 21.

The date in my answer (21 March 325 AD) corresponds to the Proleptic Gregorian Calendar:

March 21, 325 AD is a date determined by the Julian calendar. When converted retroactively to the Gregorian calendar, this date  becomes March 20, 325 AD  .

Reason: The Gregorian calendar came into effect in 1582, but when calculating backwards, a calendar correction (a day skip) was applied until 46 BC. By 325,  there was a 1-day  difference between the Julian and Gregorian calendars.

“If the goal is to fix that date, then shouldn’t the Julian and Proleptic Gregorian dates remain the same on that date?”

This is a perfectly reasonable and pertinent question. The answer lies in the fact that the aim of the reform  was to fix astronomical events  ,   not calendar dates .

1. Targeted Fixed Point:  The goal was not to fix the paper on which “March 21” was written, but  to fix the actual astronomical moment of the Spring Equinox  . At the Council of Nicaea, the spring equinox fell on March 21 (Julian). This  reflected astronomical reality at the time the Council convened  .
2. Root of the Problem:  The Julian calendar (which assumes a year of 365.25 days) was slightly longer than the true solar year (approximately 365.2422 days). This small difference ( ~11 minutes ) accumulated over the centuries, shifting the calendar away from astronomical events by about 1 day  every 128 years  .
3. The Logic of the Reformation:  In 1582, the Pope erased this accumulated error of 10 days, aligning the calendar  to the current astronomical position  . That is, after October 4, 1582, he jumped to October 15, 1582, so that the equinox fell around March 21.
4. Why is Proleptic Application Different?  The Proleptic Gregorian calendar applies this correction rule (the leap year rule change) to the beginning of history. Therefore, when we look back to the year 325, the Julian calendar was already slightly “advanced” (by one day) at that time. By retrospective calculation,  we see that the true astronomical equinox in 325  fell on March 20th according to the Gregorian calendar.

In short:  The reform aimed to correct the error in 1582 so  that future  March 21sts would coincide with the astronomical equinox. It used March 21st of 325 as a historical reference point, but this did not mean retroactively changing that date itself. The goal was to make the calendar accurate “as it was in 325,” which meant dropping 10 days in 1582.

APPENDIX 10: META

1. SUBJECT: CONQUEST OF İSTANBUL

The date of the conquest of İstanbul, May 29, 1453, is recorded according to the Julian calendar system. Although this date is accepted as May 29, 1453, according to the Gregorian calendar, the Gregorian calendar was not implemented until 1582, and therefore was not in use in 1453.

The Hijri calendar equivalent of this date is 20 Jumada al-Akhir 857.

It corresponds to Tuesday, May 29, 1453.

This date appears in Ottoman and Byzantine sources. Ottoman records kept under the orders of Mehmed the Conqueror and the works of Byzantine historians such as Kritovoulos are important sources for determining this date.

• TOPIC: THE JULIAN AND GREGORIEN CALENDARS

The Julian calendar is a solar calendar implemented by Julius Caesar in 45 BC. In this calendar, a year is considered to be an average of 365.25 days. This average was maintained by adding a day to February (February 29) every four years. However, this system incurred an error of one day every 128 years.

The Gregorian calendar was introduced in 1582 by Pope Gregory XIII to correct this error in the Julian calendar. The average year length in this calendar is 365.2425 days. This calendar has an error of three days every 400 years.

The Proleptic Gregorian Calendar is an extension of the Gregorian calendar into the past. This calendar is based on the application of Gregorian rules to dates before the official adoption of the Gregorian calendar.

The equivalents of some dates in the Julian calendar in the Proleptic Gregorian calendar and the characteristics of these days:

• January 1, 4713 BC (Julian): January 1, 4713 BC (Proleptic Gregorian) – Monday (This date is the starting point of the Julian Day Count)
• January 1, 45 BC (Julian): January 2, 45 BC (Proleptic Gregorian) – Saturday (beginning of the Julian calendar)
• January 1, 1 AD (Julian): January 3, 1 AD (Proleptic Gregorian) – Saturday
• March 21, 325 AD (Julian): March 21, 325 (Proleptic Gregorian) – Thursday (date of the Council of Nicaea)
• May 29, 1453 AD (Julian): May 29, 1453 (Proleptic Gregorian) – Tuesday (Conquest of İstanbul)
• AD 4 October 1582 (Julian): October 15, 1582 (Proleptic Gregorian) – Friday (date the Gregorian calendar began)

The Julian Day Number (JDN) is a day counting system. It expresses the number of days since January 1, 4713 BC (Julian).

The JDN number for the Gregorian date November 26, 2025 is 2461011.

META RESPONSE FOR NEW PROMPT

The Gregorian reform   was made to fix the date of  21 March 325 (Council of Nicaea), so the correct option is c .
In the proleptic Gregorian calendar, this date  remains 21 March 325  , because those dates had not yet differentiated between the two calendars.

If “fixing that date” meant ensuring that the Julian and Gregorian calendars were the same day on that day, then yes, they should have remained the same on that date. 

APPENDIX 11: CALENDAR CHAT WITH GEMINI

This was a remarkably comprehensive conversation, exploring the incredible depth and diversity of calendar systems throughout history. We explored the mathematical, cultural, and technological aspects of humanity’s efforts to measure, organize, and interpret time.

Here is a broad summary of this long-running conversation, organized chronologically and thematically:

I. The Foundations of the Western Calendar and Its Mathematical Bridge

Our conversation focused on the fundamental differences between the Julian and Gregorian calendars , the core of modern timekeeping . The Julian calendar adopted a year length of 365.25 days , applying a simple leap year rule every four years . However, this rule deviated slightly from the true astronomical year (the Tropical Year), leading to a 10-day error in the calendar by the 16th century AD.

To correct this error, the Gregorian Calendar , instituted by Pope Gregory XIII in 1582, incorporated amendments such as excluding the beginning of a century (such as 1700, 1800, and 1900) as leap years, tying the calendar more closely to the astronomical cycle. This reform initially required a 10-day shift (making October 5th the 15th). Today, for communities that do not follow Western leap year rules, this difference has reached 13-14 days . To mathematically reconcile the dates between these different systems, we examined the concept of the Julian Day Number (JDN), which serves as the reference point for all calendars .

II. Reforms of the Ottoman and Republican Periods

Turkey’s transformation of its calendar and time system was one of the most striking examples of cultural transition between West and East. The Ottoman Empire used the lunar calendar for religious purposes and the solar-based Rumi calendar for fiscal and administrative purposes . By 1917, the Rumi calendar had moved closer to the Gregorian calendar in terms of leap years, while the full reform was completed in 1926 with the official adoption of the Gregorian calendar.

The most important cultural change occurred in the hour system :

• Adhan (Turkish Time): This system, which assumed sunset at 12:00 PM, required daily adjustments, and had a practical advantage. Our discussion concluded that the adhan was the most accessible and centralized signal used to correct time during periods without electricity and communication.
• Mean Time (Alafranga): Modern time system based on the 24-hour format.

As a social reflection of this transition, we analyzed your observation from the 1960s: the continued use of the Adhan time by the very elderly demonstrated that the reform created a cultural gap between generations. Ultimately, we observed that with the proliferation of digital clocks , the Turkish habit of expressing evening hours in a 24-hour format , such as “twenty” instead of “eight,” had become ingrained in our language.

III. Origins of Ancient Calendars and Global Heritage

We find the roots of calendar history in the civilizations of Ancient Egypt and Mesopotamia .

• Egypt: Used the 365-day Civil Calendar (no leap year), inheriting the basic 365-day structure of the modern solar calendar. Egyptian astronomical knowledge directly inspired the Roman astronomer Sosigenes, architect of the Julian Calendar.
• Mesopotamia and Babylonia: They used a lunisolar calendar (lunisolar). Their most enduring legacy was their sexagesimal system , which divided an hour into 60 minutes, a minute into 60 seconds, and a circle into 360 degrees . Modern-day time measurement is based on this Babylonian legacy.
• Iran: The Jalali Calendar was a system developed under the leadership of Omar Khayyam in the 11th century, based on observation and even more precise than the Gregorian, accepting the Spring Equinox (Nevruz) as the beginning of the year.
• Ancient Greece: Developed complex, localized systems where each city-state used its own lunisolar calendar.

IV. Original Systems: Central Asia and America

We concluded our discussion with calendar systems that are completely independent of Western and Middle Eastern systems:

• The Central Asian Turkic Calendar: The Twelve Animal Calendar was a cyclical system based on 12-year cycles ( Müçel ), focusing on nomadic life and practice. The beginning of the year was generally spring, and days, months, and years were named after animals.
• Civilizations of America:
  • Maya/Aztec: They created one of the world’s most advanced calendars. The Calendar Wheel system, which combined the 260-day Tzolk’in and 365-day Haab calendars to create 52-year cycles, was prominent.
  • Inca: They used a lunar-solar based system that focused on agricultural needs and used astronomical observation posts (Sukanka) .
• Africa: Outside of Egypt, the most notable system was the Ethiopian Calendar , which adopted the Julian leap year rule and was 7-8 years behind the West .

Ultimately, we conclude that calendar systems throughout history have been the most tangible evidence of humanity’s relationship with the universe, not just counting days but also profoundly shaping culture, economy, beliefs, and even everyday language. Thank you for this detailed and chronological journey.