Leap year calculator
Is it a leap year, and which of the three clauses decided it? The rule is not “every four years”, and the shortened version is right for every year anybody has tested it on. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
It is not “every four years”
The rule has three clauses and you need all three:
- A year divisible by 4 is a leap year…
- …unless it is divisible by 100, in which case it is not…
- …unless it is also divisible by 400, in which case it is after all.
So 1900 was not a leap year and 2000 was. Those two are the only test that separates the real rule from the one everybody remembers — and between 1901 and 2099 the shortened rule gives the right answer every single time, which is why so much software carries it and nobody notices. 2100 is when the bill arrives.
Why the calendar is built this way
The Earth takes about 365.2422 days to go round the Sun. Adding a day every four years makes the average year 365.25, which overshoots by about eleven minutes a year — the Julian calendar's error, and it had accumulated to ten days by 1582. Skipping three leap years every four centuries brings the average to 365.2425, which is the Gregorian year.
That is still not exact. The remaining error is about 27 seconds a year, so the Gregorian calendar gains a whole day on the Sun roughly every 3,200 years. Nobody has decided what to do about it, and nobody needs to.
Born on 29 February
There is no legal answer to when a leapling's birthday falls in a common year, and different jurisdictions have taken different views — some say 28 February, some say 1 March. The tool above says which years you get a real one.
The missing eleven days
Britain and its colonies changed from the Julian to the Gregorian calendar in September 1752, and the day after Wednesday 2 September was Thursday 14 September. Those eleven days did not happen. Catholic Europe had changed in 1582, Russia not until 1918, and Greece not until 1923 — which is why the October Revolution happened in November. This calculator uses the Gregorian rule for every year, including years before anybody was using it, which is called the proleptic Gregorian calendar and is what ISO 8601 specifies.
What it will not do
- It will not tell you what any particular country's calendar said before it adopted the Gregorian one. See above.
- It will not handle the Julian, Hebrew, Islamic or any other calendar. Those are different questions with different rules.
- It will not know about leap seconds, which are a separate thing entirely and have nothing to do with leap years — see the timestamp converter.