Check any year, see the next leap years, and the century rule that fools everyone
| Rule (applied in order) | Year | Result |
|---|
The leap year rule everyone knows — every four years — is only two-thirds of the actual rule, and the missing third (century years skip it unless divisible by 400) has broken real software, real contracts and real birthdays. This checker applies the full Gregorian rule to any year, shows its reasoning step by step, and covers the Feb 29 practicalities: what happens to leap-day birthdays, licenses, payroll and interest when the calendar's rarest date is involved.
So 2024 and 2028: leap. 1900: not (÷100 but not ÷400). 2000: leap (÷400 — the exception's exception, and the reason the Y2K generation never noticed the century rule). 2100: not a leap year — the next time the trap springs, and "%4" code written today will quietly serve wrong Februaries that year.
Earth's orbit takes ~365.2422 days — the awkward remainder that all calendar drama serves. Julius Caesar's every-4-years fix (365.25 average) overshoots by 11 minutes a year, which compounded to 10 full days by 1582; Pope Gregory's reform deleted those days outright (October 4 was followed by October 15) and added the century rule, landing the average at 365.2425 — one day of drift per ~3,200 years. Britain and its colonies held out until 1752 (deleting 11 days, to genuine public anger), Russia until 1918 — which is why the "October" Revolution happened in November.
| Situation | How it resolves |
|---|---|
| Leap-day birthdays (~1 in 1,461 people) | Legal age in non-leap years: most US states default to March 1; the UK too; New Zealand says Feb 28 — it genuinely varies |
| Licenses & documents expiring Feb 29 | Issuing systems use Feb 28 or Mar 1 in common years; renew early rather than test the DMV's edge cases |
| Annual salaries in a 366-day year | Salaried pay doesn't change — leap-year Feb 29 is statistically a free workday for employers (the internet relitigates this every four years) |
| Daily interest (365 vs 366 denominators) | Convention-dependent: actual/365, actual/360, actual/actual all exist; on big balances the leap-day difference is real money in institutional finance |
The This Year button answers instantly. The quick mental version: divisible by 4 and not a century year → leap. 2024, 2028, 2032, 2036 lead the current run; 2026 and 2027 are common years.
Because the solar year is 365.2422 days, not 365.25 — every-4-years overcorrects by ~11 minutes/year. Skipping 3 leap days per 400 years (the century rule) trims the average to 365.2425, within half a minute of reality. The remaining error accumulates to a day only after ~3,200 years — someone else's problem.
US states mostly resolve Feb 29 birthdays to March 1 for legal-age purposes (a few statutes say Feb 28); practically, DMVs and bars follow the printed license logic. Leap-day babies ('leaplings') celebrate whichever adjacent day they prefer — the law only cares at age thresholds.
They were deleted by decree to resync the calendar with the sun: October 5-14, 1582 never existed in Catholic Europe; September 3-13, 1752 never existed in Britain/America. Contracts, rents and birthdays crossing the gap caused genuine chaos — and George Washington's birthday moved 11 days retroactively.
Yes, for anything still running — and embedded systems routinely outlive predictions (banking COBOL from the 1970s still processes trillions). The 2000 case ACCIDENTALLY worked for lazy code (÷400 = leap), so the last real-world test of the century rule was 1900. Date libraries handle it; hand-rolled date math is the risk.
All solar-tracking ones must: the Islamic calendar (pure lunar) drifts through the seasons by design; the Hebrew and Chinese calendars add leap MONTHS on 19-year cycles; the Persian calendar's astronomical rule is arguably more accurate than the Gregorian. Every calendar is a different compromise with the same 365.2422.
Yes — the check is pure arithmetic in your browser; nothing is transmitted.
Divisible by 4, minus the centuries, plus the 400s — a three-line rule holding the calendar within a day of the sun for three millennia. Check any year, win the trivia argument, and never hardcode %4.