Leap Years Explained: Why February Has 29 Days

Every four years, February picks up an extra day and a small group of people finally get to celebrate their birthday on the right date. Most of us know the basic rule: if the year divides by four, it's a leap year. That's nearly right. It's also the reason the year 2100 is going to confuse a lot of people.

Here's what's actually going on.

The problem: a year isn't 365 days

A calendar year has 365 days. The Earth doesn't care. The time it takes to go once around the seasons, from one spring equinox to the next, is about 365.2422 days. That's roughly 365 days, 5 hours and 49 minutes.

Those extra hours don't sound like much, but they add up. Ignore them and the calendar slips by almost a full day every four years. After a century you'd be out by more than three weeks. Give it a few hundred years and you'd be celebrating midsummer in the snow.

A leap day is a patch. Every so often we add a day to let the calendar catch up with the Earth.

The first fix: one extra day every four years

The idea goes back to Julius Caesar. His calendar, introduced in 45 BC, added a day every four years, which makes the average year exactly 365.25 days long.

That's very close to the real figure, but it's slightly too long, by about 11 minutes a year. Again, not much. But over centuries it piled up, and by the 1500s the calendar had drifted around ten days away from the seasons. That mattered to the church in particular, because the date of Easter depends on the spring equinox, and the equinox was arriving earlier and earlier on the calendar.

The second fix: the rule most people don't know

In 1582 Pope Gregory XIII introduced the calendar most of the world uses today. It did two things.

First, it removed the drift that had already built up. In the countries that switched straight away, Thursday 4 October 1582 was followed by Friday 15 October. Ten days simply never happened.

Second, it tweaked the leap year rule so the problem wouldn't come back. The full rule is:

A year that divides by 4 is a leap year,unless it divides by 100, in which case it isn't,unless it also divides by 400, in which case it is.

So 1900 wasn't a leap year, because it divides by 100 but not by 400. The year 2000 was a leap year, because it divides by 400. And 2100, 2200 and 2300 won't be leap years, even though they divide neatly by four.

Dropping three leap days every 400 years brings the average calendar year down to 365.2425 days. That's within about 27 seconds of the real thing. It will take roughly 3,000 years for the error to add up to a single day, which is a problem for someone else.

Not everyone switched at once

Catholic countries moved to the new calendar in 1582. Others took their time. Britain and its colonies, including what became the United States, held out until 1752. By then the gap had grown to 11 days, so Wednesday 2 September 1752 was followed by Thursday 14 September. Russia didn't change until after the 1917 revolution, which is why the "October Revolution" is marked in November.

This is also why some Orthodox churches still celebrate Christmas on 7 January. They kept the older calendar for religious dates, and it is now 13 days behind.

When is the next leap year?

The next leap day is 29 February 2028, a Tuesday. After that come 2032, 2036 and 2040. You can follow the live countdown to Leap Day 2028 if you like the idea of watching an extra day arrive.

A quick way to check any year in your head: look at the last two digits. If they divide by four, it's a leap year. Only years ending in 00 need the extra check against 400.

Being born on 29 February

People born on a leap day are sometimes called leaplings or leapers. The chance of being born on 29 February is about one in 1,461, since that's how many days there are in a four-year cycle.

In the years without a 29th, leaplings usually celebrate on 28 February or 1 March. It's not just preference, either. Some places set a legal answer for things like when you turn 18. In the UK, for example, a leapling's legal birthday in a common year is treated as 1 March. Plenty of leaplings make the most of it and celebrate across both days.

And yes, the jokes are inevitable. Someone born on 29 February 2000 turned 24 in 2024, but only had their sixth "real" birthday.

Leap day traditions

Because 29 February is a bit of an oddity, it's collected some traditions of its own. The best known in Britain and Ireland is the idea that women can propose to men on a leap day, a custom often linked, without much evidence, to a deal struck by St Bridget in Ireland. In some countries it's considered an unlucky day to get married, while in others couples pick it on purpose for the novelty of an anniversary that only comes round every four years.

A leap year isn't the only fix

For completeness, you might also hear about leap seconds. They're a separate thing, added now and then to keep clocks in line with the Earth's slightly irregular rotation, not with its orbit around the sun. They don't change the calendar, and most people never notice them.

The leap day, on the other hand, is something we all live through. It's a small correction, quietly applied every four years, that keeps December in winter and June in summer for those of us in the north. Not bad for one extra square on the calendar.