Why we need leap years
A calendar year should match the time it takes for the seasons to repeat, called the tropical year. That year lasts about 365.2422 days, or roughly 365 days, 5 hours and 49 minutes.
A calendar of exactly 365 days would fall behind the seasons by almost a quarter of a day every year. After a century it would be off by about 24 days, and spring would slowly drift into what the calendar calls winter. Adding one extra day in most years divisible by four keeps the calendar and the seasons together.
The rule
The Gregorian calendar, used in most of the world, has a three-part rule:
- A year divisible by 4 is a leap year,
- except a year divisible by 100, which is not a leap year,
- except a year divisible by 400, which is a leap year after all.
| Year | Divisible by 4? | By 100? | By 400? | Leap year? |
|---|---|---|---|---|
| 2026 | No | No | No | No |
| 2028 | Yes | No | No | Yes |
| 1900 | Yes | Yes | No | No |
| 2000 | Yes | Yes | Yes | Yes |
| 2100 | Yes | Yes | No | No |
Upcoming leap years include 2028, 2032, 2036 and 2040. The extra day is always 29 February, which makes February 29 days long instead of 28.
From Julius Caesar to Pope Gregory
The idea goes back to the Julian calendar, introduced by Julius Caesar in 46 BC. It added a leap day every four years without exception, giving an average year of 365.25 days. That was very close, but still about 11 minutes too long. Over the centuries the error added up, and by the 16th century the calendar was about ten days out of step with the seasons, which mattered to the church for setting the date of Easter.
In 1582, Pope Gregory XIII introduced the reformed calendar now named after him. It dropped ten days at once, so Thursday 4 October 1582 was followed by Friday 15 October 1582, and it added the century rule above. Its average year is 365.2425 days, which is off from the tropical year by only about one day in roughly 3,000 years.
Countries adopted the new calendar at different times. Catholic countries such as Spain, Portugal and Italy switched in 1582. Britain and its colonies, including those in North America, did not switch until 1752, by which point 11 days had to be removed: Wednesday 2 September 1752 was followed by Thursday 14 September.
How to check a year yourself
To find out whether a year is a leap year:
- If the last two digits are not divisible by 4, it is not a leap year.
- If they are divisible by 4 and the year does not end in 00, it is a leap year.
- If it ends in 00, check whether the whole year is divisible by 400.
For example, 2024 ends in 24, which is divisible by 4, so it was a leap year. 2026 ends in 26, which is not, so it is a common year.
Effects of a leap year
A leap year has 366 days and 52 weeks plus two days, so a date after 29 February falls two weekdays later than it did the year before, instead of one. This shift is part of what makes calendars repeat in a pattern of 6, 11 or 28 years.
Leap years also matter for calculations. A year from 1 March 2027 ends on 29 February 2028, the day before 1 March, and counts 366 days. Our days between dates calculator and date calculator handle leap days automatically, and the weekday calculator shows which day of the week any date falls on.
Leap year vs leap second
A leap year should not be confused with a leap second. The leap year follows a fixed rule to keep the calendar aligned with the seasons. A leap second has been added irregularly to keep atomic clocks aligned with the Earth’s rotation.