How Leap Years Work (and Why 1900 Wasn't One)

Published October 07, 2026 · By the YearsAgoCalculator team

A solar year lasts about 365.2422 days. The calendar keeps up with that extra quarter day by adding February 29 in leap years, using three rules.

The three rules

1. A year divisible by 4 is a leap year.
2. Except a year divisible by 100, which is not.
3. Except a year divisible by 400, which is a leap year after all.

So 2024 and 2028 are leap years, 1900 and 2100 are not, and 2000 was. Over 400 years this gives 97 leap days and an average year of 365.2425 days, very close to the real solar year.

Why it matters for date math

Leap days change day counts between dates and shift weekdays. Someone born on February 29 has a calendar birthday only once every four years in most cases.

Check any year with the leap year calculator.

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