Leap Year Checker

How to use the leap year checker

1. Enter a year.
2. Read the answer and the reasoning.
3. Use the nearby years to see the pattern.

About this tool

A year is a leap year if it divides by 4 — except years divisible by 100, which are not, unless they also divide by 400, which are. So 1900 was not a leap year and 2000 was.

That second exception is the one that matters, because a great deal of software has ignored it. Code checking only for divisibility by four was correct for every year between 1901 and 2099, which is why the bug survived so long undetected and then surfaced in 2000 in systems that had run correctly for decades.

The reason for the correction is that a year is not 365.25 days but about 365.2422. Adding a day every four years overshoots by roughly 11 minutes a year, which accumulates to a full day every 128 years or so. Skipping three leap days every four centuries removes most of that drift, leaving an error of about one day in 3,200 years.

That residual is why 29 February is a genuinely awkward date in software: it exists in some years and not others, which breaks any code that assumes a date can be reconstructed a year later.

Every calculation is done in UTC. Working in local time silently gains or loses an hour across a daylight saving boundary, which turns a difference of exactly seven days into six days and twenty-three hours — a bug that appears twice a year and is therefore rarely caught.

Common uses

• Checking whether a year has 29 February.
• Confirming the century rule for a historical date.
• Explaining why the leap year rule has exceptions.

Frequently asked questions

Was 1900 a leap year?

No. It divides by 100 but not by 400, so it was skipped. 2000 divides by 400 and was a leap year.

Why is the rule not just every four years?

A year is about 365.2422 days, not 365.25. Adding a day every four years overshoots, so three leap days are skipped every four centuries to correct it.

Will 2100 be a leap year?

No. It divides by 100 but not by 400, so it will be skipped — the first time that has happened since 1900.