Why isn't every 4th year a leap year?

Century years (like 1900) are only leap years if divisible by 400, which is why 2000 was a leap year but 1900 was not.

Leap Year Checker

Check whether any year is a leap year using the standard Gregorian calendar rule.

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Formula

Leap if divisible by 4, except century years not divisible by 400

Calculating Time Duration Across Midnight, and the Real Leap Year Rule

How to calculate elapsed time when it wraps past midnight, and the three-part rule that actually governs leap years.

Day of the Week Calculator

Find out what day of the week any date falls on.

Age Calculator

Calculate your exact age in years, months, and days.

Date Difference Calculator

Calculate the number of days, weeks, and months between two dates.

Countdown Calculator

Calculate the time remaining until a future date and time.

Time Duration Calculator

Calculate the duration between two clock times in hours and minutes.

Week Number Calculator

Find the ISO week number for any date.