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Leap Second

Also called: UTC leap second

Definition: A leap second is a one-second adjustment added to Coordinated Universal Time (UTC) so it stays within 0.9 s of Earth-rotation time, giving a day of 86,401 seconds.

Updated November 4, 2026 · How we check facts

A leap second keeps civil time (UTC) aligned with the rotation of the Earth. The IERS Earth Orientation Center decides when to insert one so that UT1 − UTC stays below 0.9 second. On that day the last minute has 61 seconds, as on 30 June 2012, so the day has 86,401 seconds instead of 86,400.

Example

A normal day: 24 × 3,600 = 86,400 s. A leap-second day: 86,400 + 1 = 86,401 s. The IERS page counts 27 leap seconds added between 1972 and its most recent one on 1 January 2017. Leap seconds are announced in the IERS Bulletin C. See how many seconds in a day.

Common mistakes

The SI second and the exact conversion day = 86,400 s are unaffected. Only UTC clock time includes the extra second.

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Free toolDate Duration CalculatorDays between two dates, or a date plus N days.

Guides that explain it

Related terms

Second (s)The second is the SI base unit of time, defined from the caesium-133 transition frequency of 9,192,631,770 Hz; there are exactly 60 seconds in a minute and 86,400 in a day.Sidereal DayA sidereal day is one rotation of the Earth relative to the stars: about 86,164.09 seconds, or 23 h 56 min 4.09 s, roughly 3 min 56 s shorter than a 24-hour solar day.Gregorian CalendarThe Gregorian calendar is the civil calendar in worldwide use. Its 400-year cycle has 146,097 days, an average year of 365.2425 days, kept by the leap-year rule.Hour (h)The hour is a unit of time equal to exactly 60 minutes or 3,600 seconds, and one 24th of a day. It is accepted for use with the SI but is not an SI unit itself.Leap YearA leap year has 366 days instead of 365. In the Gregorian calendar a year is a leap year if divisible by 4, except centurial years that are not divisible by 400.Minute (min)The minute is a unit of time equal to exactly 60 seconds, one 60th of an hour. It is accepted for use with the SI, and a day has 1,440 of them.Tropical YearThe tropical year is the time between successive spring equinoxes, about 365.2422 days (about 31,556,930 seconds). It is the natural year the Gregorian calendar approximates.

Sources

  1. IERS Earth Orientation Center — The leap second