How to use Leap Year Checker online, free

A year is 365 days, except the Earth takes about 365.2422 days to orbit the Sun. Ignore that quarter-day and the calendar drifts against the seasons by roughly 24 days per century - which is exactly what happened under the Julian calendar, until by the 1500s the spring equinox had slid ten days out of place. The Gregorian reform fixed it with a three-part rule that is more subtle than most people remember. A year is a leap year if divisible by 4, unless it is also divisible by 100, unless it is further divisible by 400. That is why 2000 was a leap year and 1900 was not, despite both dividing neatly by 4 and by 100. The second exception is the part that gets forgotten, and it caused real software failures in 2000 when systems implemented only the first two rules. The remaining error is about one day in 3,000 years. Checks run in your browser.

Date maths goes wrong when time zones, leap years or "today" vs a chosen date get mixed up. Set the inputs deliberately, then trust the output.

Leap Year Checker is a good fit when confirming whether February has 29 days in a particular year.

Why this exists

Leap Year Checker is built around a few practical wins, not a long feature list:

• Applies all three rules, including the 400-year exception most people and some software forget.
• Explains the reasoning rather than returning a bare yes or no.
• Works for any year, historical or far future.
• Runs in your browser with no account.
• Useful for checking date logic in code as well as everyday questions.

Walkthrough

1. Enter a year. Any year, past or future. The Gregorian rules apply from 1582 onwards, when the calendar was adopted.
2. Read the answer. Leap year or not, with the reasoning shown rather than just a yes or no.
3. Follow the rule chain. Divisible by 4, then the 100 exception, then the 400 exception - seeing which applied is the useful part.

Jobs it is built for

• Confirming whether February has 29 days in a particular year.
• Checking date handling when writing or testing software.
• Working out how a 29 February birthday or anniversary falls.
• Settling the question for a historical date.
• Explaining the rule to someone who has only heard the "divisible by 4" version.

Worth knowing before you start

• The full rule is three parts, not one. Divisible by 4, except centuries, except centuries divisible by 400.
• 2000 was a leap year; 1900 and 2100 are not. Those are the examples that make the rule stick.
• When writing date code, use your language's date library rather than implementing the rule yourself - that is where the 2000 bugs came from.
• Leap seconds are a completely separate mechanism for a different problem, and are not part of leap year rules.
• For dates before 1582, the Gregorian rules are applied retrospectively rather than describing what any calendar actually did at the time.

What not to expect

• Remembering only "divisible by 4", which gets 1900 and 2100 wrong.
• Implementing the rule by hand in code instead of using a tested date library.
• Assuming every century year is skipped, which misses 1600, 2000 and 2400.
• Confusing leap years with leap seconds, which correct for the Earth's rotation rather than its orbit.
• Forgetting 29 February exists when validating date input, which is a persistent source of form bugs.

Privacy, in one paragraph

Leap Year Checker runs in your browser. The file or text you paste stays on your device. There is no account, and nothing is stored on a ToolBox server for this job.

Related tools worth opening next

If this is one step in a longer job, these usually come after it:

• Date Difference - Find the exact number of days, months and years between two dates
• Calendar Generator - View and export a printable monthly or yearly calendar

Short answers

What are the actual rules for determining a leap year?

A year is a leap year if it's divisible by 4, EXCEPT if it's also divisible by 100 - unless it's also divisible by 400, in which case it IS a leap year after all. This is why 2000 was a leap year but 1900 was not, despite both being divisible by 100.

Why do leap years exist at all?

Earth's orbit around the sun takes approximately 365.25 days, not exactly 365 - leap years add an extra day roughly every 4 years to keep our calendar synchronized with the actual solar year over long periods.

Can I check any year, including far future or past years?

Yes. Note that the Gregorian rules were adopted in 1582, so for earlier years the answer applies the modern rule retrospectively rather than describing the calendar in use at the time.

Why was 2000 a leap year but 1900 was not?

Both are divisible by 4 and by 100, which would normally exclude them. But 2000 is also divisible by 400, and that third rule puts it back in. This is the part of the rule most often forgotten - and it caused genuine software failures in 2000 for systems that implemented only the first two conditions.

How accurate is the Gregorian calendar?

The three-rule system leaves an error of roughly one day in 3,000 years. The Julian calendar it replaced drifted about 24 days per century, which is why the equinox had slipped ten days out of place by the time of the reform.

What happens to a 29 February birthday in non-leap years?

There is no legal universal answer, and it varies by jurisdiction and organisation. Most people mark it on 28 February or 1 March, and different systems handle the edge case differently - which is itself a common source of software bugs.

Open the Leap Year Checker when you are ready. It is free, and you do not need an account.