The rule
In the words of the US Naval Observatory: years evenly divisible by 4 are leap years, with the exception of centurial years that are not evenly divisible by 400. Three questions, in order:
- Is the year divisible by 4? If not, it is a common year of 365 days.
- Is it divisible by 100? If not, it is a leap year.
- Is it divisible by 400? If yes, it is a leap year after all; if not, it is a common year.
| Year | Divisible by 4 | Divisible by 100 | Divisible by 400 | Leap year? |
|---|---|---|---|---|
| 1900 | yes | yes | no | no |
| 1996 | yes | no | no | yes |
| 2000 | yes | yes | yes | yes |
| 2024 | yes | no | no | yes |
| 2025 | no | no | no | no |
| 2026 | no | no | no | no |
| 2028 | yes | no | no | yes |
| 2100 | yes | yes | no | no |
| 2400 | yes | yes | yes | yes |
The next leap years are 2024, 2028, 2032, 2036, 2040, 2044, 2048, 2052; 2100 is the next skipped one. The days between dates calculator and the age calculator apply the rule automatically, which is the only safe way to count days across a February.
Where the rule comes from
The Earth's trip round the Sun, measured from one spring equinox to the next, takes about 365.2422 days. A calendar of 365 days therefore falls behind the seasons by almost a quarter of a day each year. The Julian calendar fixed that with a leap day every four years, giving an average year of 365.25 days, slightly too long: the seasons crept earlier by about 0.78 days per century. The Naval Observatory describes the consequence: by 1582 the drift was large enough that Pope Gregory XIII shifted the calendar so that spring again began on March 21, and replaced the every-fourth-year rule with 97 leap days in every 400 years. That is the 400-year exception.
| Calendar | Leap days per 400 years | Average year | Drift against the tropical year |
|---|---|---|---|
| Julian | 100 | 365.25 days | about 0.78 days per century |
| Gregorian | 97 | 365.2425 days | about 0.03 days per century, or one day in roughly 3000 years |
The Gregorian average of 365.2425 days is still a touch longer than 365.2422 days, so the calendar gains about a day every three millennia. No fix is scheduled; the tropical year itself changes slowly, and the error is small enough to leave to a future century.
What the extra day does
- Weekdays shift by two. A common year is 52 weeks and one day, so a date moves one weekday forward from one year to the next. A leap year is 52 weeks and two days, so dates after February move two weekdays. That is why the day of the week finder needs the leap rule, and why a calendar repeats exactly only every 28 years in the stretches without a skipped century.
- Ages and anniversaries. Someone born on February 29 has a calendar birthday every four years; most systems and the age calculator count the birthday as February 28 or March 1 in other years, and the age in years is unaffected.
- Day counts. A year from March 1, 2027 is 366 days in 2028 but 365 in other years; interest, deadlines and subscriptions that count days rather than months are a day off from what a glance at the calendar suggests. The days between dates calculator counts the real days.
- Software. The classic bug is implementing only "divisible by 4". It passes every test until a century year arrives; 2000 happened to be a leap year by both rules, so the next time the mistake shows is 2100.
Other calendars, briefly
The Gregorian rule is the civil standard worldwide, but it is not the only one in use. The Julian calendar, still used for some religious dates, keeps every fourth year and has drifted almost two weeks behind. Lunar and lunisolar calendars add a whole leap month rather than a day. For any date conversion between systems, the Julian date converter works with the astronomers' day count, which sidesteps calendars entirely by numbering the days.
Frequently asked questions
What is the leap year rule?
A year is a leap year if it is divisible by 4, except that century years are leap years only if they are divisible by 400. The US Naval Observatory states it exactly that way: years evenly divisible by 4 are leap years, with the exception of centurial years that are not evenly divisible by 400. So 2024 and 2000 are leap years; 1900 and 2100 are not.
Why is 2100 not a leap year?
Because it is a century year not divisible by 400. The extra day every four years slightly overcorrects: the Julian calendar's average of 365.25 days is longer than the tropical year of about 365.2422 days. Skipping three leap days every 400 years brings the average to 365.2425 days, which is why 1700, 1800, 1900 and 2100 are skipped while 1600, 2000 and 2400 keep theirs.
When is the next leap year?
2024, 2028, 2032, 2036 and then every four years until 2100, which is skipped. February 29 falls on a different weekday each time, so plans made "on the 29th" repeat only every 28 years at most.
Why does a leap year exist at all?
The Earth takes about 365.2422 days to go round the Sun. A 365-day calendar would drift against the seasons by almost a day every four years, so a day is added. The US Naval Observatory explains that by 1582 the Julian calendar had drifted enough that Pope Gregory XIII shifted the date to bring the spring equinox back to March 21 and introduced the 400-year rule.
How many days are in a leap year?
366, with February having 29 days instead of 28. Everything else is unchanged, which is why a date in March or later lands one weekday further on than it would have without the extra day, and why date arithmetic has to know which years are leap years.