Leap year rules: why 2100 has no February 29
The exception to the exception explains February’s missing day—and why repeating the calendar every four years does not quite work.
Three tests, in the right order
For the Gregorian calendar, a year divisible by 4 is usually a leap year. But a year divisible by 100 is not—unless it is also divisible by 400. That last exception restores 2000 as a leap year while excluding 1900 and 2100. It is easiest to check 400 first, then 100, then 4. Divisibility is about the full year number, not its last two digits: the ending “00” alone cannot settle the question.
What the rule buys us
In 400 years, the simple every-four-years rule would insert 100 leap days. The century rule removes three of them, leaving 97. The total is 400 × 365 + 97 = 146,097 days, or an average of 365.2425 days per year. That average is much closer to the seasonal year than 365.25 days. A civil calendar does not need every year to have the same length; it needs a workable pattern that keeps dates broadly aligned with the seasons.
February birthdays reveal a second choice
Someone born on 29 February has a perfectly valid birth date. In a common year, however, a “next birthday” feature needs a convention: 28 February or 1 March. The leap-year rule does not decide that convention. Our age tool uses 28 February for its next-birthday calculation. That is a disclosed calculation choice, not a universal statement about how someone celebrates or which date a legal rule uses. Calendar arithmetic and birthday customs are different layers.
Test the years that catch shortcuts
A useful test set is 2024, 2025, 1900, 2000, and 2100. Their February lengths are 29, 28, 28, 29, and 28 days. Testing only 2024 and 2025 would let an incomplete “divisible by four” implementation pass. You can inspect each in our year calendar. For historical dates, remember that applying Gregorian rules backward is a calculation convention; different places adopted this calendar at different times. A modern calendar grid is not proof of the dating system used in an old document.
Year explorer
Choose a year to test the Gregorian rule.
Test your intuition
Sources & further reading
Original explanations with worked examples. Reference links point to the underlying standards and documentation.