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Divisible by 4 = leap year, EXCEPT century years, which must be divisible by 400: so 2000 was a leap year but 1900 wasn't and 2100 won't be. The correction exists because the solar year is 365.2422 days, not 365.25: three skipped leap days every 400 years closes most of the gap.
What The Leap Year Calculator Works Out
- Leap year?: the Gregorian test. A year divisible by 4 is a leap year, unless it is a century year, in which case it must also be divisible by 400. So 2000 was a leap year, 1900 was not and 2100 will not be.
- Next leap years: the following four leap years under the same rule, so you can see the gaps (always four years apart except around skipped centuries, where the gap is eight).
- The rule: the three-line test in plain words. It exists because the solar year is 365.2422 days, not 365.25: Julius Caesar's every-fourth-year fix drifted about 11 minutes a year, and by 1582 the equinox had wandered ten days. Britain adopted the fix in 1752 by deleting eleven September days.
- Calendar rule (advanced): switch to the Julian rule (every fourth year, no century exception) to see how the old calendar treats the same year. Under the Julian rule 1900 and 2100 are leap years.
- Days in the year (advanced): 366 or 365, plus the weekday that 29 February (or 1 March in a common year) falls on. Weekdays are proleptic Gregorian, so dates before 1752 in Britain used a different calendar.
- Leap years in your range (advanced): a count of leap years between two years inclusive, with the total days in the span. Every 400 Gregorian years contain exactly 97 leap years (146,097 days, which is also exactly 20,871 weeks, so the calendar repeats).
- Leap days you have lived through (advanced): the 29 Februaries from your birth year to the year checked. Someone born in 1990 has seen nine by 2027. Leaplings (born on 29 February, about 1 in 1,461) legally age on 1 March in England for milestones such as turning 18.
- Calendar drift (advanced): how far each rule's average year is from the real solar year. Gregorian (365.2425) drifts about one day every 3,300 years; Julian (365.25) drifts a day every 128 years, which is why the two calendars are now 13 days apart.
- How many upcoming leap years to list (advanced): extend the list up to twelve years ahead, handy for checking a century boundary such as 2096, 2104 with no leap year in between.