How to use it
- The current year is checked when the page opens.
- Type any other year in the Year field and press Check, or just hit Enter.
- The chips name the next leap year and the previous one, so you can count forward or back without doing the arithmetic.
The rule, in full
A year is a leap year if it divides by 4, unless it divides by 100, unless it also divides by 400. The short version taught in school stops at the first clause and gets centuries wrong: 1900 was not a leap year and 2100 will not be, while 2000 was. This page applies all three clauses, which is why you can trust it on a century boundary.
The correction exists because the Earth takes about 365.2422 days to go round the Sun, not 365.25. Adding a day every four years overshoots by roughly 11 minutes a year; skipping three leap days every four centuries brings the calendar back to within about half a minute a year. That is accurate enough to drift a single day over three thousand years.
Where it matters in practice
- A 29 February birthday, and what happens to it in the other three years.
- Contracts and interest counted in actual days: a leap year has 366.
- Date code. Adding a year to 29 February 2024 lands on 1 March 2025 in most libraries, which is correct and still surprising.
Questions
Is this year a leap year?
The page answers when it opens: YES or NO, with the day count of the year next to it.
Why was 1900 not a leap year?
Because it divides by 100 but not by 400, and the Gregorian rule drops those century years. 1600 and 2000 kept their leap day; 1700, 1800 and 1900 did not.
How many days does a leap year have?
366, with the extra one on 29 February. A normal year has 365.
How far does the check reach?
Any year from 1 to 9999, using the Gregorian rule throughout. For years before 1582 that rule is applied retroactively, which is the usual convention in software.
