Leap Year Checker

A leap year is a year that contains 366 days instead of the usual 365, with the extra day added as February 29th. Leap years exist because the Earth's orbital period around the Sun is approximately 365.2422 days — not exactly 365. Without leap years, the calendar would drift by about 24 days every century, eventually causing summer months to fall in winter. The Gregorian calendar (introduced in 1582 by Pope Gregory XIII) uses a specific set of rules to keep the calendar synchronized with the astronomical year.

The leap year rules are more nuanced than simply 'every 4 years.' Years divisible by 100 are NOT leap years, unless they are also divisible by 400. This means 1900 was not a leap year (divisible by 100 but not 400), while 2000 WAS a leap year (divisible by 400). This century rule reduces the average calendar year to 365.2425 days, very close to the true orbital period. Software developers must implement these rules correctly to avoid bugs in date calculations, financial systems, and scheduling applications — the famous Y2K bug was partly a leap year issue.

The probability of being born on February 29 is approximately 1 in 1,461 (0.068%). People born on Feb 29 are called 'leaplings' or 'leapers' — they typically celebrate their birthday on Feb 28 or Mar 1 in non-leap years. The Gregorian calendar's leap year system will accumulate a 1-day error approximately every 3,236 years. Some countries historically used different calendar systems with different leap year rules. In programming, most standard library date functions handle leap years automatically, but custom implementations often get the century rule wrong.

When building software, always use standard library date functions rather than implementing leap year logic manually. Test your code with edge cases: year 2000 (leap), 1900 (not leap), 2100 (not leap), 2400 (leap). When scheduling events on February 29, have a fallback for non-leap years. Financial calculations spanning leap years must account for the extra day in interest calculations. When validating user-entered dates, check for February 29 in non-leap years.