Leap Years Explained

What a leap year is

A leap year is a year with 366 days instead of 365. The extra day is 29 February, called a leap day. Leap years keep the calendar in step with the seasons, because the Earth takes about 365 and a quarter days to orbit the sun, and a calendar of exactly 365 days would drift against the seasons by about a day every four years.

The common leap years in recent history are 2020, 2024 and 2028. The next ones after 2026 are 2028, 2032 and 2036.

The rule in three lines

A year is a leap year if it is divisible by 4. But if it is divisible by 100, it is not. Unless it is also divisible by 400, in which case it is.

So 2024 is a leap year, because it divides by 4. 1900 was not, because it divides by 100 but not by 400. 2000 was, because it divides by 400. 2100 will not be, and 2400 will be.

The rule is easy to remember as: every fourth year, except the century years that cannot be divided by 400. Among the years from 1901 to 2099, every fourth year is a leap year with no exceptions, which is why the simple rule works for almost all everyday dates.

Why the calendar needs leap years

A solar year, the time the Earth takes to return to the same point in its orbit relative to the seasons, is about 365.2422 days. A calendar year of 365 days is short by almost a quarter of a day, and the difference adds up to a day every four years.

The Julian calendar, introduced by Julius Caesar in 45 BC, added a leap day every fourth year, giving an average year of 365.25 days. That is slightly too long, and over centuries the calendar drifted ahead of the seasons. By the sixteenth century the difference was about ten days.

Pope Gregory XIII reformed the calendar in 1582 by dropping three leap years in every 400, which is the century rule above. The average Gregorian year is 365.2425 days, because there are 97 leap years in each 400-year cycle. The remaining error is about one day in several thousand years.

How leap years change date calculations

A leap day adds one day to any span that contains it. From 28 February 2028 to 1 March 2028 is 2 days, while the same dates in 2026 are 1 day apart. A year from 1 March 2027 to 1 March 2028 has 366 days, because it contains 29 February 2028.

Leap years also change the weekday of dates. An ordinary year is 52 weeks and 1 day, so a date moves forward one weekday from one year to the next. A leap year is 52 weeks and 2 days, so dates after 29 February move forward two weekdays. That is why 22 October 2026 falls on a Thursday and 22 October 2027 on a Friday, and 22 October 2028 falls on a Sunday.

When you add one year to a date, the result is the same day and month one year later. When you add 365 days, the result is a day earlier if a leap day was in between. If a rule says a year, use a year.

Leap-day birthdays and anniversaries

People born on 29 February are sometimes called leaplings. In ordinary years their birthday has no matching date, and different places and organisations treat it differently, with 28 February and 1 March both common. The calculator on this site uses 28 February.

The effect on the exact age is small. Age counts complete years and months from the date of birth, and the leap-day rule only decides which day in an ordinary year counts as the birthday.

Anniversaries and contracts that start on 29 February face the same question. A yearly payment dated 29 February might fall due on the 28th or the 1st in an ordinary year, and the agreement should say which.

Misconceptions and edge cases

It is not true that every year divisible by 4 is a leap year. The century rule exists to remove three of them every four hundred years. A calculator or a spreadsheet that ignores it will be right for dates between 1901 and 2099 and wrong for 1900 and 2100.

Spreadsheets have a famous quirk here. Microsoft Excel treats 1900 as a leap year, which it was not, for compatibility with an older program. A date in January or February 1900 can therefore be off by a day in a spreadsheet.

A leap second, an occasional one-second correction to clocks, is a different thing. It adjusts the time of day, not the calendar, and it does not affect a date calculator.

Using the calculator

Enter any date on the Day of the week tab, and the calculator shows whether its year is a leap year, together with the days in the month and the days left in the year.

To see the effect of a leap day on a span, compare two ranges on the days-between tab. From 1 March 2027 to 1 March 2028 is 366 days, and from 1 March 2026 to 1 March 2027 is 365.