A leap year has 366 days instead of 365. The extra date is February 29, inserted between February 28 and March 1. That one day keeps the civil calendar close to the seasonal year, but it also changes weekday patterns, payroll counts, anniversaries and every date after February.
The quick rule “every four years” works most of the time. It fails at certain century years, which is why a complete answer needs three tests rather than one.
The Gregorian leap-year rule
Use the tests in this order:
- If the year is not divisible by 4, it is a common year.
- If it is divisible by 4 but not by 100, it is a leap year.
- If it is divisible by 100, it must also be divisible by 400 to remain a leap year.
The result is easy to check with examples. The year 2024 is divisible by 4 and not by 100, so it was a leap year. The year 2026 is not divisible by 4, so it is a common year. The year 1900 is divisible by both 4 and 100 but not 400, so February 1900 had 28 days. The year 2000 is divisible by 400, so February 2000 had 29 days. The year 2100 will again be a common year.
In code, the rule can be expressed as: divisible by 4, and either not divisible by 100 or divisible by 400. Old Calendars uses that test throughout the calendar engine rather than maintaining a hand-entered list of leap years.
Why the calendar needs an extra day
The seasonal, or tropical, year is not exactly 365 days. The US Naval Observatory gives it as approximately 365.2422 days. If every civil year contained only 365 days, the calendar would lose almost a quarter of a day against the seasons each year. The mismatch would build to roughly 24 days over a century.
Adding one day every four years produces an average of 365.25 days. That is much closer, but still slightly too long. Over several centuries, the small excess matters. The Gregorian reform removes three leap days in each 400-year span by treating century years as common years unless they are divisible by 400.
There are therefore 97 leap years in a 400-year Gregorian cycle, not 100. The average civil year becomes 365.2425 days. It is not a perfect physical measurement of Earth’s orbit, but it holds the calendar close to the seasonal cycle over ordinary historical timescales.
This is also why “every four years forever” is not a safe rule for spreadsheet formulas, family-history software or a same-calendar calculator. A formula that marks 2100 as a leap year will be wrong for every date after February 28 that year.
What February 29 changes inside the year
January and February begin on the same weekdays in a leap year and a common year that share the same January 1. After February 29, the leap-year calendar is one weekday ahead.
Suppose March 1 is Friday in a common year. With the same starting weekday and an inserted February 29, March 1 would be Saturday in the leap version. That shift continues through December 31. It affects weekday counts, business-day totals, holiday positions and which months contain five Mondays or five Fridays.
A leap year also moves the next New Year’s Day by two weekdays. A common year contains 52 full weeks plus one day; a leap year contains 52 full weeks plus two days. This is one reason whole calendars do not simply repeat every seven years.
The 2020 calendar and 2024 calendar both include February 29, but they do not match because January 1 fell on different weekdays. The same-calendar year finder compares both leap status and starting weekday.
Century years are the useful stress test
When reviewing a calendar app, test 1900, 2000 and 2100. Together they reveal whether the implementation uses the complete Gregorian rule.
- February 29, 1900 is invalid in the Gregorian calendar.
- February 29, 2000 is valid.
- February 29, 2100 will be invalid.
Some spreadsheet programs preserve a historical compatibility bug that treats 1900 as a leap year. That behavior was designed to match earlier software, not the Gregorian calendar. When serial date numbers move between systems, dates around February and March 1900 deserve special checking.
Old Calendars’s public archive begins in 1995, so it does not display 1900 as a year page. Its calculators extend the Gregorian rules across a broader range, and the leap-year test still includes the century exception.
February 29 birthdays and anniversaries
Someone born on February 29 has a birthday date that appears only in leap years, but the person still ages every year. In a common year, people commonly celebrate on February 28 or March 1. Preference, family tradition and local law can point to different choices.
The legal detail matters for age thresholds, contracts and benefits. A statute may define when a person born on February 29 reaches a particular age in a non-leap year. Calendar arithmetic alone cannot supply a universal legal answer, so the age calculator should be used as a convenient date calculation rather than legal advice.
Anniversary software also needs a stated rule. An application may display February 28 as the last day of the birth month, or March 1 as the day after February 28. Neither choice changes the original date. Good software keeps February 29 in the record and treats the common-year reminder as a display policy.
Leap day is not leap second
The similar names describe different corrections. A leap day belongs to a predictable calendar rule. It adds a complete civil date, February 29, to selected years.
A leap second concerns timekeeping at the level of seconds. It has been used to keep atomic time close to time based on Earth’s rotation. Leap seconds are not scheduled by the divisible-by-4 rule, and they do not create a new date. A calendar can calculate leap years centuries ahead without knowing future leap-second decisions.
The distinction matters when software documentation simply says it “handles leap time.” Date logic, time-zone logic and precision timekeeping are separate jobs and should be tested separately.
How to check a leap year without guessing
For an ordinary year, divide by 4. If there is a remainder, stop: it is a common year. If the division is exact and the year does not end in 00, it is a leap year.
For a century year, divide by 400. An exact result means leap year; a remainder means common year. That short routine works for the full Gregorian cycle.
After checking the rule, open the complete calendar and confirm three visible consequences: February has 29 days, March begins one weekday later than it would in a common year, and the total length is 366 days. Old Calendars lists all three facts on every year page, alongside weekday counts and working-day totals.
For a date-range calculation that crosses February, the days-between-dates calculator counts the actual dates rather than assuming every year has 365 days. That is safer than multiplying years by 365 or using an average such as 365.25 for an exact civil deadline.
Frequently asked questions
How do I know whether a year is a leap year?
A Gregorian year is a leap year if it is divisible by 4, except that a year divisible by 100 is not a leap year unless it is also divisible by 400. That makes 2000 a leap year but 1900 and 2100 common years.
Why was 2000 a leap year but 1900 was not?
Both years are divisible by 100, so the ordinary divisible-by-4 test is not enough. The century exception restores leap status only when the year is divisible by 400. Two thousand passes that test; 1900 does not.
How many days are in a leap year?
A leap year has 366 days. February receives the extra day and has 29 days instead of 28. The other eleven months keep their usual lengths.
When do people born on February 29 celebrate in a common year?
There is no single universal rule. Many people choose February 28 or March 1 socially. Laws and contracts can define the legal anniversary differently by jurisdiction, so an official age or deadline question should be checked locally.
Is a leap second the same as a leap day?
No. A leap day is a calendar correction inserted according to a fixed Gregorian rule. A leap second is a timekeeping adjustment connected with Earth's rotation and atomic time; it does not add a date to the calendar.
Sources
- Leap Years - US Naval Observatory
- Introduction to Calendars - US Naval Observatory
- Explanatory Supplement to the Astronomical Almanac - US Naval Observatory
Calculated examples were checked against the Old Calendars date engine. See our editorial policy for how we research and correct pages.