Calendars and dates
Leap years explained: why 2000 was one and 2100 will not be
A leap year exists because Earth does not orbit the sun in a whole number of days. The rule that handles the leftover fraction is short, slightly odd, and accurate to about a day in three thousand years.
Last reviewed October 5, 2026About 3 min to readSite version v2026.10.1
The problem: a year is not 365 days
The time from one March equinox to the next, the tropical year, is about 365.2422 days. A calendar of exactly 365 days would run ahead of the seasons by almost a quarter of a day every year, so after 120 years the equinox would arrive a month late. Something has to absorb the fraction.
Julius Caesar's calendar of 45 BC did it by adding one day every four years, making the average year 365.25 days. That is close, but it overshoots by about 11 minutes a year. By the sixteenth century the accumulated error was ten days, and the equinox that the church used to fix the date of Easter had drifted to March 11.
The Gregorian rule
The reform of 1582 kept the four-year cycle and removed three leap days every four hundred years to cancel the overshoot. The rule has three parts, applied in order:
- A year divisible by 4 is a leap year,
- unless it is divisible by 100, in which case it is not,
- unless it is also divisible by 400, in which case it is.
| Year | Divisible by 4? | By 100? | By 400? | Leap year? |
|---|---|---|---|---|
| 1900 | Yes | Yes | No | No |
| 2000 | Yes | Yes | Yes | Yes |
| 2024 | Yes | No | Yes | |
| 2026 | No | No | ||
| 2100 | Yes | Yes | No | No |
The result is 97 leap years every 400 years and an average year of 365.2425 days, which is within about 27 seconds of the tropical year. The remaining error adds up to roughly one day every 3,000 years, and no correction for it has been agreed, because nobody alive will need one.
When the days went missing
Catholic countries adopted the new calendar immediately: in Italy, Spain, Portugal and Poland, Thursday October 4, 1582 was followed by Friday October 15. Other countries held out for centuries. Britain and its colonies, including the future United States, switched in September 1752, when Wednesday September 2 was followed by Thursday September 14, eleven days later because the Julian calendar had slipped one more day by then. Russia changed in 1918, after the revolution, and Greece in 1923.
Those gaps matter when you compute historical date differences. George Washington's birthday is recorded as February 11, 1731 under the calendar in force when he was born and February 22, 1732 under the one adopted during his lifetime; both are the same day. The days between dates calculator works in the proleptic Gregorian calendar, which applies today's rule backward through history, and says so, because that is the convention software uses and the one that produces consistent answers.
Practical edges of the rule
- February 29 birthdays. Someone born on February 29, 2024 next sees their exact date in 2028. The add or subtract a date tool shows what "one year after February 29" resolves to, since calendars disagree on whether that is February 28 or March 1.
- Day counts. A leap year has 366 days, so a date range that spans February 29 is one day longer than the same range a year earlier. Payroll, interest and age calculations that assume 365 drift by a day each leap year.
- Day of the week. Because 366 is two more than a multiple of seven, a leap year shifts the weekday of every date after February by two days instead of one. The day of the week calculator handles that for any date.
- Leap seconds are unrelated. A leap second is a one-second adjustment to UTC, not to the calendar, and is explained in the UTC vs GMT guide.
See it on the site
- Days between datesAn exact day count that includes every February 29 in the range.
- Day of the weekThe weekday of any date, past or future, under the Gregorian rule.
- 2028 calendarThe next leap year, with February 29 in place.