No. 2100 is divisible by 100 but not by 400, so under the Gregorian rule it is a common year of 365 days — even though it divides by 4.

How often is a leap year?

Almost every four years. The only exceptions are century years not divisible by 400 (like 1900 and 2100), which skip the leap day to keep the calendar accurate.

What happens to a 29 February birthday in a common year?

There is no 29 February in a common year, so the birthday is usually marked on 28 February or 1 March. Exact-age math still counts the days correctly either way.

What Is a Leap Year? The Rule, the Reason, and How to Check

Written & reviewed by Ramanathan · Last updated September 9, 2026

A leap year is a year with 366 days instead of the usual 365, created by adding an extra day — 29 February — to the calendar. That single day keeps our calendar lined up with the time it actually takes the Earth to travel once around the Sun. Without it, the seasons would slowly drift out of place, and after a few centuries our summer months would fall in winter.

The leap year rule

A year is a leap year if it is divisible by 4 — with one exception. Century years (those ending in 00) are not leap years unless they are also divisible by 400.

So the full rule is:

Divisible by 4? It might be a leap year.
Also divisible by 100? Then it is not a leap year…
…unless it is also divisible by 400, in which case it is.

That is why 2000 was a leap year (divisible by 400) but 1900 and 2100 are not (divisible by 100 but not 400). Ordinary years like 2024 and 2028 are leap years simply because they divide by 4.

Why we need leap years

The Earth takes about 365.2422 days to orbit the Sun — not a neat 365. If every calendar year were exactly 365 days, we would lose almost a quarter of a day each year. Over four years that adds up to nearly a full day, and over a century to around 24 days.

Adding one extra day every four years puts back most of that lost time. The extra correction for century years fine-tunes the average, bringing the calendar year to 365.2425 days — close enough that it stays accurate to within one day for thousands of years.

From the Julian to the Gregorian calendar

The idea of a leap year is old. The Julian calendar, introduced by Julius Caesar in 45 BCE, added a leap day every four years without exception. That slightly overcorrected, and by the 1500s the calendar had drifted about ten days out of step with the seasons.

In 1582 the Gregorian calendar fixed this by adding the century-year exception (the divisible-by-100-and-400 rule) and dropping ten days to realign the date. It is the calendar almost the whole world uses today, and it is the one every calculation on this site follows.

How to check whether a year is a leap year

You can apply the rule by hand: divide the year by 4; if it is a century year, check 400 instead. Or you can let a tool do it for you. Our leap year calculator checks any year instantly, shows the reason for the answer, and tells you the next leap year — handy when you are planning around 29 February.

Leap day birthdays and day counts

People born on 29 February — sometimes called leaplings — only see their exact birthday once every four years. In common years their birthday is usually observed on 28 February or 1 March. When you work out an exact age, the day still counts normally: our age calculator treats someone born on 29 February 2004 as exactly 20 on 29 February 2024.

Because every calculator here counts real calendar days, leap days are already built into the results — a span that crosses a 29 February is one day longer, with no special handling needed on your part.