Why Do We Have Leap Years? The Rule and the Next One

We have leap years because the seasons repeat every 365.2422 days or so, a little longer than the 365 days in a normal calendar year (USNO). Without an extra day now and then, the calendar would slip about 24 days behind the seasons every century (same source). Adding February 29 in most years divisible by 4 keeps the two lined up. The next leap day is Tuesday, February 29, 2028.

The leap year rule

The modern Gregorian calendar uses one rule with one exception. In the U.S. Naval Observatory’s words, years evenly divisible by 4 are leap years, “with the exception of centurial years that are not evenly divisible by 400” (USNO).

To check any year:

1. If it doesn’t divide evenly by 4, it’s a normal year. 2026 and 2027 are normal years.
2. If it divides by 4 but isn’t a century year, it’s a leap year. 2028 is one.
3. If it’s a century year, it’s a leap year only if it also divides by 400. 2000 was a leap year. 1700, 1800 and 1900 were not, and 2100 won’t be (USNO).

The century exception drops three leap days every 400 years (NIST). That leaves 97 leap years in each 400-year cycle, so the calendar year averages 365.2425 days, very close to the 365.2422 days of the seasons.

Why every fourth year wasn’t enough

The simple “every fourth year” rule comes from Julius Caesar’s calendar, the Julian calendar. The Naval Observatory’s leap year page dates it to 46 BC, and its calendars page dates it to 45 BC (USNO, USNO calendars). Royal Museums Greenwich also gives 46 BC (Royal Museums Greenwich). So official sources differ by a year, and two of the three pages here give 46 BC.

A leap day every fourth year makes the average year 365.25 days. That’s about 11 minutes longer than the seasonal year, and the error builds to roughly three-quarters of a day per century. By the 1500s, the start of spring had drifted from March 23 to March 11 (USNO).

Pope Gregory XIII fixed this in 1582. He added the century exception and shifted the calendar so that spring would again begin around March 21 (USNO). To make the shift, the day after October 4, 1582 became October 15, 1582 (NASA). Ten dates never happened that year.

Britain and its colonies, including the ones that became the United States, changed over in 1752, when September 2 was followed by September 14 (Royal Museums Greenwich, NASA). By then the gap had grown to 11 days.

How accurate is the calendar now?

The Gregorian year agrees with the seasons to within about half a minute, and it will take about 3,300 years for the calendar to drift a full day (USNO).

Upcoming leap years

Leap day	Day of the week
February 29, 2028	Tuesday
February 29, 2032	Sunday
February 29, 2036	Friday
February 29, 2040	Wednesday

The next time the century rule kicks in is 2100, which won’t have a February 29 (USNO).

Leap day birthdays and traditions

People born on February 29 might be called leapers, leaplings or leap day babies (Library of Congress). They get their actual birthday only once every four years, though they age the same as everyone else (NIST).

The best-known leap day custom is about marriage proposals. According to an Irish tradition, a leap year was a chance for women to propose to men, either on February 29 or at any point that year (Library of Congress). The Library of Congress post collects old newspaper stories about the custom.

Leap seconds are a different thing

A leap second was a single second added to Coordinated Universal Time (UTC), the world’s atomic time scale, to keep it close to time measured by Earth’s rotation, which is slightly irregular (NIST). It has nothing to do with the length of the year. The first leap second was added on June 30, 1972, and NIST’s table lists 27 in all. The most recent one came on December 31, 2016 (NIST).

Leap seconds are now being phased out. In 2022 the General Conference on Weights and Measures (CGPM) decided to loosen the limit that made leap seconds necessary “in, or before, 2035.” At its October 2026 meeting it set the date: UTC becomes continuous on May 20, 2027, and its gap from Earth-rotation time may grow to as much as an hour before any correction (BIPM). The 2026 resolution is the newer decision, so it replaces the “by 2035” timeline you may still see quoted.

Leap days aren’t affected. February 29 will keep turning up every four years, with the century exception, for as long as we use the Gregorian calendar.

For more dates worth knowing, see our list of famous historical dates.