Calendar rule guide
Last reviewed: August 2026 ยท By Age in Days editorial desk
A leap year is divisible by 4, except centuries must also be divisible by 400.
That means 1900 and 2100 are not leap years, while 2000 and 2400 are. Exact age tools need this exception to decide whether February 29 is a valid date.
The rule in practice
February has 29 days in a year divisible by 4. A year divisible by 100 loses that extra day unless it is also divisible by 400. This keeps the Gregorian calendar aligned with the seasonal year more closely than a simple every-four-years rule.
Worked examples
1900-02-29 is invalid because 1900 is a century not divisible by 400. 2000-02-29 is valid because 2000 is divisible by 400. From February 28, 2024 to March 1, 2024 there are two calendar days; in 2025, the same month/day labels have one day between them.
February 29 birthdays
The date of birth stays February 29. When showing an informational next birthday in a non-leap year, this site uses February 28. Other contexts can choose March 1, and a law, contract, school, or agency can apply its own rule. The calculator does not give legal advice.
Common implementation failures
Bad date parsers accept February 29 in 1900 and silently roll it into March. Another error is treating every fourth year as leap, which fails at 2100. A third is using age times 365 and omitting leap days altogether.
Try the next birthday calculator, compare the February 29 birthday guide, and consult the methodology. The calendar rule is summarized by the timeanddate leap-year reference.