Most people know the shortcut: leap years happen every four years. That rule is right about 96% of the time, and the 4% it gets wrong is the interesting part — it's the reason 1900 had no 29 February and 2000 did, and the reason a lot of otherwise careful date arithmetic quietly produces the wrong answer.
The actual rule
A year is a leap year if it is divisible by 4, except years divisible by 100, unless they are also divisible by 400.
| Year | ÷ 4 | ÷ 100 | ÷ 400 | Leap year? |
|---|---|---|---|---|
| 2024 | Yes | No | — | Yes |
| 2025 | No | — | — | No |
| 1900 | Yes | Yes | No | No |
| 2000 | Yes | Yes | Yes | Yes |
| 2100 | Yes | Yes | No | No |
| 2400 | Yes | Yes | Yes | Yes |
Anyone who wrote date-handling code between 1901 and 1999 could use the simple divisible-by-four rule and never be wrong, because every multiple of four in that window is a leap year. A great deal of software was written on that assumption, and 2000 passed without incident precisely because it was a leap year — the naive rule accidentally gave the right answer. The next real test is 2100, which is not a leap year despite being divisible by four.
Why the correction exists
The Earth takes about 365.2422 days to orbit the sun. A 365-day calendar therefore drifts by roughly a quarter of a day each year — after four years, nearly a full day.
The Julian calendar, introduced in 46 BC, fixed this by adding a day every four years, giving an average year of exactly 365.25 days. That overcorrects slightly: 365.25 is about 11 minutes longer than the true solar year. Eleven minutes a year is nothing, but over centuries it accumulates. By the 1500s the calendar had drifted about ten days out of step with the seasons, which mattered because the date of Easter is calculated from the spring equinox.
The Gregorian reform of 1582 did two things. It deleted ten days to realign the calendar — in Catholic Europe, 4 October 1582 was followed directly by 15 October. And it added the century exception, skipping three leap days every 400 years. That brings the average calendar year to 365.2425 days, within about 27 seconds of the solar year. The remaining drift amounts to roughly one day every 3,200 years.
Britain and its colonies, including what became the United States, did not adopt the Gregorian calendar until 1752, by which point eleven days had to be dropped. This is why some historical dates are written in both "Old Style" and "New Style," and why George Washington's birthday is recorded as both 11 February and 22 February 1732.
Leap years and the days of the week
A common year is 365 days — 52 weeks plus one day. So a date normally advances one weekday per year: if 1 March falls on a Friday this year, it falls on a Saturday next year. A leap year is 366 days, or 52 weeks plus two days, so any date after 29 February advances by two weekdays instead of one.
The full pattern of calendar and weekday alignment repeats every 400 years. One consequence of that cycle, easy to verify and mildly satisfying: the 13th of the month falls on a Friday slightly more often than on any other weekday.
For the weekday of a specific date, the born on what day tool works it out directly, including for dates that fall on a leap day.
People born on 29 February
Roughly one person in 1,461 is born on a leap day — about five million people worldwide. Three quarters of the time, their birth date does not exist in the calendar.
There is no universal convention for when a "leapling" has a birthday in common years, and different systems answer differently:
- 28 February is the more common choice for legal and administrative purposes in the UK, Hong Kong and New Zealand — the reasoning being that the person turns a year older at the end of February.
- 1 March is the convention in much of the United States and in Taiwan, on the reasoning that a full year has not elapsed until the day after 28 February.
- Both, in practice, for most families.
For age arithmetic this matters less than it seems. Age in completed years is normally computed by comparing whole years, so a person born 29 February 2000 is 26 on 1 March 2026 either way. The ambiguity only affects the single day of 28 February. The age calculator here handles leap-day births explicitly rather than silently rounding.
Leap seconds are a different thing entirely
Leap years correct the calendar against the Earth's orbit. Leap seconds correct clocks against the Earth's rotation, which is gradually slowing and does so irregularly. Since 1972, 27 leap seconds have been added to UTC, always at the end of June or December, announced only about six months in advance.
Unlike leap years they follow no predictable rule — they are declared as needed, based on measurements. They have also caused real outages, because a minute containing 61 seconds breaks software that assumes it cannot. In 2022 the international body responsible voted to stop inserting them by 2035.
Checking a year yourself
In plain terms: if the year ends in two zeros, it must be divisible by 400 to be a leap year. Otherwise, divisible by 4 is enough.
Leap years this century that catch people out: none, until 2100. Every multiple of four from 2004 through 2096 is a leap year. Then 2100, 2200 and 2300 are skipped, and 2400 is a leap year again.