Leap years explained: the rules, the dates and the edge cases

Every fourth year the calendar gains an extra day, 29 February, and a few oddities come with it. Why does it exist? Why was 1900 not a leap year even though it divides by four? What happens to someone born on 29 February, and does a leap year change your pay or your tenancy? This guide explains the astronomy in plain terms, gives the complete rule with a list of leap years up to 2048, works through some examples, and covers what the extra day means in everyday life in the UK, from birthdays to salaries to contracts that run for a year.

By Valtrinity. Updated 18 October 2026.

Why we need a leap day

The Earth takes about 365.2422 days to go once round the Sun. A calendar of exactly 365 days would fall behind the seasons by almost a quarter of a day every year. After four years, it would be about a day out, and after a century, about 24 days.

Adding a day every fourth year fixes most of the drift. Four years of 0.2422 extra days add up to 0.9688 days, which is close to one. But it slightly overcorrects, by about 0.03 of a day every four years, which is why the full rule has two exceptions.

The system we use now is the Gregorian calendar, introduced in 1582 and adopted in Britain and its colonies in 1752. Britain moved from the older Julian calendar at that point and dropped 11 days, so that 2 September 1752 was followed by 14 September.

The rule in three parts

A year is a leap year if it is divisible by 4. There is an exception: if the year is divisible by 100, it is not a leap year. There is an exception to the exception: if the year is divisible by 400, it is a leap year after all.

Apply it to some examples. 2024 is divisible by 4 and not by 100, so it is a leap year. 2026 is not divisible by 4, so it is not. 1900 is divisible by 4 and by 100 but not by 400, so it was not a leap year. 2000 is divisible by 400, so it was. 2100 will not be, and 2400 will be.

The average year in this system is 365.2425 days, which is very close to the true value of 365.2422. The difference builds up to one day only after about 3,000 years.

Leap years from 2024 to 2048

The most recent leap year was 2024. The next ones are 2028, 2032, 2036, 2040, 2044 and 2048. They come every four years, and there is no century exception to worry about until 2100.

So 2026 and 2027 are ordinary years of 365 days, with February ending on the 28th. In 2028, February has 29 days. If you are planning something that runs across February 2028, such as a project or a contract, include the extra day in your count.

A quick mental check is to take the last two digits. If they divide by four, it is a leap year, unless they are 00, in which case the whole year must divide by 400. For example, 36 divides by 4, so 2036 is a leap year. 50 does not, so 2050 is not.

Birthdays on 29 February

About one person in 1,461 is born on 29 February, which is roughly one in every four years of births. They are sometimes called leaplings. They have a true birthday only in leap years, so the question is what to do the rest of the time.

In everyday life people simply celebrate on 28 February or on 1 March, whichever they prefer. Where an exact date matters, such as the day someone turns 18 or reaches a pension age, rules can treat the birthday in a non-leap year as either 28 February or 1 March, so check the official guidance for your own situation rather than assuming.

Someone born on 29 February 2000 has had only six true birthdays by 2026: 2004, 2008, 2012, 2016, 2020 and 2024. Yet they count as 26 years old in 2026, because age counts years since birth, whether or not the date exists in each year.

What a leap year means for pay and money

Most salaries are received monthly and do not change in a leap year. The annual figure is divided by 12, so February 2028 pays the same as any other month.

Daily or hourly arrangements can be different. Someone on a daily rate will have one extra weekday in some leap years, depending on which day of the week 29 February falls on. In 2028, 29 February falls on a Tuesday, so there is one more working day than in a normal February.

Interest is more complex. Savings and loans quote an annual rate, and some calculate daily interest by dividing by 365 even in a leap year, and some use 366. The terms and conditions of the account say which. The difference is tiny. On £10,000 at 5 per cent, the interest is £500 a year, which is £1.37 a day when divided by 365 and £1.37 a day when divided by 366, rounded to the penny.

Contracts, tenancies and deadlines

A 12-month contract starting on 1 March 2027 ends on 29 February 2028, the day before the anniversary. That period contains 366 days, so a contract that says 365 days instead of a year ends on 28 or 29 February, depending on whether the first day is counted. The wording decides which applies, so read it to see whether it says a year or a number of days.

For a deadline of 'one month from' a date, the standard reading is the corresponding date next month, or the last day if there is no match. One month from 29 January 2028 is 29 February 2028. One month from 29 January 2027 is 28 February 2027, because there is no 29 February that year.

Software can get the leap day wrong. Famous failures include systems that rejected 29 February dates, or that handled 2100 incorrectly. If you write or buy software that handles dates, test 29 February 2028, 28 February and 1 March in a non-leap year, and the year 2100.

Leap seconds and the bigger picture

The leap day is not the only adjustment. The Earth's rotation is slightly irregular, so since 1972 timekeepers have occasionally added a leap second to UTC, usually at the end of June or December. The last was in 2016.

Leap seconds are a different matter from the calendar, and they do not affect everyday dates. Most people will never notice them, though they can matter for computer systems that depend on precise time.

The calendar will need another adjustment eventually, since the average Gregorian year is slightly longer than the true year. That will not be a concern for many centuries.

Tools mentioned in this guide

Frequently asked questions