Daniel Nashed's Blog

Checking leap year rules in Notes -- With historic view

This is a follow-on post to my previous Notes TIMEDATE post.
To sum it up. Notes is handing it right.

Checking if the calendar rules work for dates long time ago, made me look into history behind our modern time system.
I wrote a program to check Notes leap rules leveraging the Gregorian calendar.
The only year in question I found 1700. But it turned out this really depends when the Gergorian calendar became effective.

In the Julian calendar the dividable by 400 rule was not in place. It's arguable if this is an issue. That depends on the county ..
The US changed to the Gregorian calendar in 1752. For all future dates the Gregorian rules apply and future century leap days look right in my testing.

Here is what ChatGPT found out.. But you have to read it right and it is interesting to read.
I can't recall hearing about those history details in school ... I did know the rules. But not when they had been introduced in different countries.

-- Daniel

Leap year exceptions are related to the rules of the Gregorian calendar and its predecessor, the Julian calendar. These exceptions address the need to keep the calendar year aligned with Earth's orbit around the Sun. Here is a breakdown:

Leap Year Rules

1. Julian Calendar (Introduced in 45 BCE):
   - A year is a leap year if it is divisible by 4.
   - This rule overestimated the length of the solar year, causing an error of about 11 minutes annually. Over centuries, this accumulated, leading to calendar drift.

2.Gregorian Calendar (Introduced in 1582):
   - To correct the Julian calendar's drift, the Gregorian calendar added two additional rules:
     - A year is a leap year if it is divisible by 4.
     - **Exception:** If the year is divisible by 100, it is not a leap year, unless:
     - **Exception to the exception:** If the year is divisible by 400, it is a leap year.
   - This refinement corrected the annual discrepancy and aligned the calendar more closely with the solar year.

Historical Exceptions

1. Switch from Julian to Gregorian Calendar:
   - In 1582, Pope Gregory XIII introduced the Gregorian calendar. Ten days were skipped to realign the calendar with the seasons. Different countries adopted the Gregorian calendar at different times, causing discrepancies.
   - Example: In England and its colonies, the switch occurred in 1752, skipping 11 days (September 2 was followed by September 14).

2. Non-implementation of Leap Years:
   - Leap years that would occur under the Julian rules but not under the Gregorian rules (e.g., 1700, 1800, 1900) are exceptions. These were not leap years in Gregorian calendar countries but remained so in Julian calendar countries.

3. Adoption Delays:
   - Some countries, like Russia and Greece, retained the Julian calendar for centuries after 1582. Russia adopted the Gregorian calendar in 1918, and Greece in 1923.

Rare Leap Year Events

- Year 2000:
   - While divisible by 100 (not a leap year under the basic rule), it was also divisible by 400, making it a leap year under the Gregorian rules.

Summary of Leap Year Exceptions

- 1700, 1800, 1900: Not leap years in the Gregorian calendar, but leap years in the Julian calendar.
- Year 2000: A leap year in both systems due to the "divisible by 400" rule.

These rules and exceptions ensure the calendar remains synchronized with Earth's orbit, minimizing long-term drift.

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