What is a leap second?
A leap second is an extra second added to UTC, as 23:59:60, to keep atomic time within 0.9 seconds of Earth's slightly irregular rotation. 27 have been added since 1972, the last on December 31, 2016. In 2022 the world's metrology body decided to stop adding them by 2035.
Key facts
| Leap seconds added | 27 |
|---|---|
| First | June 30, 1972 |
| Most recent | December 31, 2016 |
| TAI − UTC | 37 seconds (10 initial + 27) |
| Negative leap seconds | None so far |
| Decided by | IERS (International Earth Rotation and Reference Systems Service) |
| Ending | By 2035 (CGPM Resolution 4, 2022) |
Why they exist
Atomic clocks tick perfectly evenly; Earth doesn't spin perfectly evenly. Left alone, atomic time would slowly drift away from the sun's position in the sky. Leap seconds were introduced in 1972 so that UTC tracks the planet. When the gap approaches 0.9 seconds, the IERS announces a leap second about six months in advance, to be inserted at the end of June 30 or December 31.
What happens during one
Clocks run 23:59:58, 23:59:59, 23:59:60, then 00:00:00. That one-second minute has repeatedly broken software, including outages at websites and airline systems in 2012. Many tech companies now "smear" the extra second across a whole day instead.
All leap seconds
| Year | Added at the end of |
|---|---|
| 1972 | June 30, December 31 |
| 1973–1979 | December 31 each year |
| 1981–1983 | June 30 |
| 1985 | June 30 |
| 1987 | December 31 |
| 1989, 1990 | December 31 |
| 1992–1994 | June 30 |
| 1995 | December 31 |
| 1997 | June 30 |
| 1998 | December 31 |
| 2005 | December 31 |
| 2008 | December 31 |
| 2012 | June 30 |
| 2015 | June 30 |
| 2016 | December 31 |
Why none since 2016
Earth's rotation has recently been slightly faster than the atomic day, so no leap seconds have been needed, and scientists even discussed the first-ever negative leap second. The 2022 decision to let the gap grow larger by 2035 means that, in practice, the leap second is being retired.
Questions people ask
When was the last leap second?
December 31, 2016, at 23:59:60 UTC.
Will there be any more leap seconds?
Probably not. None are scheduled, and the 2022 CGPM decision means they will stop by 2035.
Is a leap second the same as a leap year?
No. A leap year adds a day to keep the calendar in step with Earth's orbit. A leap second adds a second to keep clocks in step with Earth's rotation.
Sources
Last updated October 18, 2026. Found an error? Tell us.