A leap day falls on February 29, giving that date birthdays only once every four years.

The longer version

Because the extra day is inserted only in years divisible by four, with the century exception, 29 February appears roughly once in four. People born then have far fewer matching anniversaries than anyone else, about one date in every 1,461. The gap can stretch to eight years around a skipped century year.

Most choose to celebrate on 28 February or 1 March in other years, and legal systems have had to decide which date counts for age. The rarity makes the date popular for publicity. People with such birthdays often joke about ageing slowly, and records offices generally accept one of the neighbouring days.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.

← Entry 46 Entry 48 →

More From Time and Calendars

Eight more entries

The Moon's cycle of roughly 29 days gave rise to the length of a month.

Many cultures began the new year in spring rather than in the middle of winter.

Chinese, Hebrew, and Islamic calendars follow lunar cycles and differ in length from the solar year.

A sidereal day is slightly shorter than 24 hours because Earth also moves along its orbit.

Standard time replaced thousands of local times, making railway schedules possible in the nineteenth century.

A day is roughly twenty-four hours because that is how long Earth takes to rotate once.

A year lasts about 365 days because Earth needs that long to orbit the Sun once.

Leap years add an extra day in February to keep calendars aligned with Earth's orbit.

Read

Try Another Subject

Three collections