What a leap year is
The solar year is about 365.2422 days, roughly a quarter-day longer than 365. To keep the calendar aligned with the seasons, a day is added at the end of February about every four years, making the year 366 days.
The full rule
A Gregorian year is a leap year if it is divisible by 4, except when divisible by 100, unless also divisible by 400.
| Year | ÷4 | ÷100 | ÷400 | Leap? |
|---|---|---|---|---|
| 2024 | yes | no | — | yes |
| 2026 | no | — | — | no |
| 1900 | yes | yes | no | no |
| 2000 | yes | yes | yes | yes |
| 2100 | yes | yes | no | no |
This makes the average Gregorian year 365.2425 days. In practice: 1 January → 1 March is 59 days in 2026 but 60 days in 2024, and 31 December is day 366 of a leap year. Check any date with Day of the Year.
When do 29 February babies get older?
In common years there is no 29 February. Two conventions exist: 28 February (last day of the same month) or 1 March (the day after). Our tools use 28 February everywhere, and apply the same rule to any day that does not exist in a month. Someone born on 29 February 2000 completes 27 years on 28 February 2027, and their next birthday after that is 29 February 2028. Some legal systems use 1 March, so check with the relevant authority for legal matters.
And the Hijri calendar?
Umm al-Qura has no leap-year rule as such: month lengths are set astronomically, giving 354- or 355-day years. The arithmetic Hijri calendar calls 355-day years “leap” and has 11 of them in every 30 years. The same last-day rule applies to someone born on the 30th of a Hijri month that later has 29 days. See The Hijri months in order, or try 29 February in the Age Calculator.
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