How leap years work

The leap year rule is three rules stacked, not one. Every fourth year gets an extra day, 29 February, making a 366-day year instead of the usual 365 — except century years, which skip it, except centuries divisible by 400, which get it back. That is why 1900 had no 29 February, why 2000 did, and why 2100 will not. The rule exists because a year is 365.2425 days long by the calendar’s own reckoning, and the calendar has to pay back that leftover fraction somehow or drift slowly away from the seasons.

The rule in full

The three layers apply in order, each one overriding the last:

• Layer one: a year divisible by 4 is a leap year. 2024 qualifies, so it ran to 366 days.
• Layer two: but a year divisible by 100 is not a leap year, even though it’s divisible by 4. This is why 1900 was an ordinary 365-day year.
• Layer three: unless the year is also divisible by 400, in which case it is a leap year after all. 2000 is divisible by 400, so it kept its 29 February.

Run the same test on 2100: divisible by 4, divisible by 100, not divisible by 400. It will not be a leap year, and 28 February 2100 will be followed straight by 1 March.

Outside the century years, the rule is simple and regular. The upcoming leap years are 2024, 2028, 2032, 2036 and 2040 — each one four years after the last, with no exceptions due until 2100.

Why one day every four years isn’t enough

The simplest possible fix — add a day every four years, no exceptions — is what the Julian calendar did, and it isn’t quite right. That rule assumes the year is exactly 365.25 days long. The real figure, in the Gregorian calendar’s own accounting, is 365.2425 days: slightly short of 365.25. The difference looks trivial, but it compounds.

The record shows that the Julian calendar, introduced in Julius Caesar’s reform of 45 BC, ran on that 365.25-day assumption and overshot the true year by about 11 minutes annually. Eleven minutes a year sounds negligible, but it adds up to roughly a day every 128 years. Left uncorrected for over a millennium, the calendar drifted noticeably out of step with the equinoxes and solstices it was meant to track.

The correction came in 1582, when Pope Gregory XIII’s reform removed three leap days every four centuries by skipping them in century years not divisible by 400 — the exceptions layered onto the original rule. That single change is what separates the Julian year’s 365.25 days from the Gregorian year’s 365.2425. Historians date the fix’s most visible moment to October 1582, when the first adopting countries jumped straight from 4 October to 15 October to erase the drift that had already accumulated. Adoption spread unevenly after that: Britain and its colonies didn’t switch until 1752, by which point they had to skip eleven days to catch up.

What the fraction buys

That 0.2425 of a day matters because a calendar’s whole job is to keep dates lined up with the seasons. Without leap days, the calendar year would run slightly short of the solar year, and the mismatch would build every single year with nothing to stop it. Given enough time, a date that currently falls in early spring would drift backward through winter, then into the depths of the preceding autumn, cycling slowly through every season over centuries. March would stop meaning early spring in the northern hemisphere and start meaning midwinter, then autumn, and so on, with no fixed relationship between the calendar page and the position of the earth in its orbit.

The leap day is the correction mechanism that stops this drift before it starts. By adding back very close to the exact shortfall — no more, no less — the calendar keeps 21 March pinned near the spring equinox indefinitely, not just for a few centuries.

The rule in one test

For any given year, the leap-year test is a single sequence of divisibility checks:

1. Is the year divisible by 4? If not, it’s an ordinary 365-day year.
2. If yes, is it also divisible by 100? If not, it’s a leap year.
3. If yes, is it also divisible by 400? If yes, it’s a leap year after all. If not, it’s ordinary.

Related ideas worth understanding alongside this one: the Gregorian calendar as the system this rule belongs to, what a leap-day birthday means for someone born on 29 February, and the date calculator for working out spans that cross leap years.