Every four years or so, February gains a 29th day, and most people know the basic rule: if the year divides by four, it is a leap year. That rule is only the first of three, though, and the other two are why the year 2000 had a leap day while 1900 did not, and why 2100 will be an ordinary year. This guide explains why the calendar needs the extra day at all, how the rules came to be, how well they work, and what all of it means for the roughly five million people alive today who were born on 29 February.
Why the calendar needs leap years
A calendar year is a promise that the seasons will arrive at the same time every year. The seasons follow the Earth’s journey around the Sun, measured from one spring equinox to the next, and that journey, called the tropical year, takes about 365.2422 days. A calendar has to use whole days, so an ordinary year of 365 days falls short by almost a quarter of a day every year.
A quarter of a day sounds trivial, but it adds up quickly. Without any correction, the calendar would slip about one day every four years, about a week each generation, and roughly a month every 120 years. After seven centuries, midsummer would arrive in what the calendar called December. Leap years exist to stop that drift by adding a day back whenever the shortfall has built up to roughly a full day.
The three rules
The modern Gregorian calendar, used for civil purposes almost everywhere, decides leap years with three tests applied in order. It helps to read them as a rule followed by two exceptions, each one finer than the last.
- A year that divides evenly by 4 is a leap year. This handles the quarter day, so 2024, 2028 and 2032 all have a 29 February.
- Except century years, which are not leap years. A year that divides by 100, such as 1900 or 2100, loses its leap day. This removes the small overcorrection that the first rule creates.
- Except century years that divide by 400, which are leap years after all. So 1600 and 2000 kept their leap days, and 2400 will too.
Put together, the rules produce 97 leap years in every 400, as the grid above shows. That gives an average year of 365 + 97/400 days, which is 365.2425 days, very close to the true tropical year. The 400-year cycle also has a neat side effect: it contains exactly 146,097 days, which is a whole number of weeks (20,871), so the calendar repeats exactly, weekdays and all, every 400 years.
From Julius Caesar to 1752
The first rule is more than two thousand years old. By the time of Julius Caesar, the Roman calendar had drifted badly out of step with the seasons, partly because the officials responsible for adding extra months did so irregularly. Advised by the astronomer Sosigenes of Alexandria, Caesar introduced a calendar in 46 BC with 365 days and a leap day every fourth year. That gave an average year of 365.25 days, much better than before but still about 11 minutes too long.
Eleven minutes a year is about one day every 128 years, and by the 16th century the Julian calendar had drifted about ten days away from the seasons. That mattered to the Church because the date of Easter depends on the spring equinox. In 1582 Pope Gregory XIII issued a papal bull, Inter gravissimas, that did two things. It removed ten days at once, so that Thursday 4 October 1582 was followed by Friday 15 October, and it added the two century exceptions to keep the drift from returning.
Catholic countries such as Spain, Portugal and the Italian states changed straight away, but many others took their time. Great Britain and its colonies, including those in North America, switched only in 1752, by which point the gap had grown to eleven days. Wednesday 2 September 1752 was followed by Thursday 14 September. Some countries changed even later: Russia adopted the Gregorian calendar in 1918, and Greece in 1923. This is why historical dates are sometimes written with “Old Style” and “New Style” versions, and why George Washington’s birthday, 11 February 1731 in the calendar of his birth, is celebrated as 22 February.
How accurate is it now?
The Gregorian average of 365.2425 days is longer than the tropical year by roughly 0.0003 days, about 27 seconds a year. That works out to an error of around one day every three thousand years or so. Proposals to fix even that, for example by skipping the leap day in years divisible by 4,000, have never been adopted, partly because the length of the tropical year itself changes slowly over thousands of years and the Earth’s rotation is gradually slowing. For any practical purpose in our lifetimes and those of our great-grandchildren, the calendar is as good as it needs to be.
Leap seconds are sometimes confused with leap days, but they solve a different problem. They keep clock time aligned with the Earth’s rotation, which wobbles slightly, rather than keeping the calendar aligned with the seasons. They have been added irregularly since 1972, and in 2022 the international body responsible for time standards voted to stop adding them by 2035.
| Calendar | Leap years per 400 | Average year | Drift |
|---|---|---|---|
| 365 days, no leap years | 0 | 365.0000 days | 1 day every 4 years |
| Julian (46 BC) | 100 | 365.2500 days | 1 day in about 128 years |
| Gregorian (1582) | 97 | 365.2425 days | 1 day in about 3,000+ years |
How many leap days have you lived?
Because every birth year between 1901 and 2099 follows the simple divide-by-four rule (2000 was a leap year, so nobody alive has lived through a skipped century year), counting your leap days is easy. Count the 29 Februarys between your date of birth and today. A person born on 12 April 1990 had lived through nine by 30 October 2026: 1992, 1996, 2000, 2004, 2008, 2012, 2016, 2020 and 2024. Those nine extra days are why their age in days is 13,350 rather than the 13,341 you would get by multiplying by 365.
Leap days are also why your birthday shifts across the week. In an ordinary year, a date moves forward one weekday, because 365 days is 52 weeks and one day. When a 29 February falls between two birthdays, the date jumps two weekdays instead. That is why your birthday seems to skip a day of the week every few years.
Born on 29 February
People born on a leap day are often called leaplings or leapers. With about one day in every 1,461 being a 29 February, and a world population a little over eight billion, a rough estimate puts their number at around five and a half million. They age exactly like everyone else: a leapling born in 2000 turned 26 in 2026. What they miss is the calendar date itself, which only comes round in leap years.
In the other years, most leaplings celebrate on 28 February or 1 March, and the choice is usually personal or family tradition. For legal matters, such as the date you become old enough to drive or vote, the rule depends on local law, and different jurisdictions have answered the question differently. If it matters for a document, check the law where you live rather than relying on a general rule.
Leap-day birthdays have also inspired a long run of jokes and stories, the most famous being the Pirates of Penzance, in which the hero discovers he was apprenticed until his 21st birthday rather than his 21st year, and so must serve until he is in his eighties. The arithmetic behind the joke is sound: someone born on 29 February 1896 had to wait eight years for the next one, because 1900, a century year, had no leap day, and the next 29 February did not come until 1904.
Leap years ahead
The next leap days fall in 2028, 2032, 2036 and 2040, and the pattern continues every four years until 2096. Then the century rule takes over: 2100 will not be a leap year, so anyone alive then will see a four-year gap become an eight-year gap between 2096 and 2104. The year 2400, like 2000, will keep its 29 February because it divides by 400.
| Year | Divides by 4? | Divides by 100? | Divides by 400? | Leap year? |
|---|---|---|---|---|
| 1900 | Yes | Yes | No | No |
| 2000 | Yes | Yes | Yes | Yes |
| 2026 | No | No | No | No |
| 2028 | Yes | No | No | Yes |
| 2100 | Yes | Yes | No | No |
| 2400 | Yes | Yes | Yes | Yes |
If you want to test any year yourself, run it through the three rules in order and stop at the first one that settles it. And if you were born on a leap day, the birthday countdown will tell you how long until your next real one.
Questions people ask
Is 2100 a leap year?
No. 2100 divides by four, but it is a century year that does not divide by 400, so it will be an ordinary year with 28 days in February. The next century leap year after 2000 is 2400.
Why is the extra day in February?
Because February was already the short month. In the early Roman calendar the year began in March, so February was the last month and the natural place to adjust the count. The habit stuck when Julius Caesar reformed the calendar in 46 BC.
How many leap years are there in 400 years?
Exactly 97. There are 100 years divisible by four in any 400-year span, and three of them are century years that do not divide by 400.
What are the odds of being born on 29 February?
Roughly 1 in 1,461, because one day in every four-year cycle of 1,461 days is a leap day. Births are not perfectly even across the calendar, so the real figure drifts a little from year to year.
Is there such a thing as a leap second?
Yes, but it is a separate fix. Leap seconds keep clock time in step with the Earth’s slightly irregular rotation, while leap days keep the calendar in step with the seasons. International timekeepers have agreed to stop adding leap seconds by 2035.
Sources. Pope Gregory XIII, papal bull Inter gravissimas (1582); Calendar (New Style) Act 1750, Great Britain; US Naval Observatory, length of the tropical year; International Bureau of Weights and Measures (BIPM), 2022 resolution on leap seconds.