Every Leap Second Since 1972

Why they're all positive

Earth's rotation is very gradually slowing due to tidal friction from the Moon, so atomic time (which doesn't care about the Earth's rotation at all) keeps pulling slightly ahead of solar time. Every leap second added so far has been a positive one — an extra second inserted to let solar time catch back up — because the rotation has only ever needed slowing down, never speeding up, relative to the atomic clock. A negative leap second (skipping a second) is technically possible under the system's rules but has never actually happened.

A pause in the schedule

As of this page's last update, the IERS had announced that no leap second would be added at the end of December 2026 — the next possible insertion date under the current schedule is June 30, 2027. Leap seconds are added irregularly, based on actual measurements of Earth's rotation, not on a fixed timetable, so this can change with future IERS bulletins.

How this affects Unix time

Unix timestamps officially ignore leap seconds entirely — every day is defined as exactly 86,400 seconds, even the days that technically had 86,401. This is exactly why Unix time isn't a true count of elapsed physical seconds since 1970, and why systems that need that precision use TAI instead. For the technical details, see GPS Time vs UTC vs Unix Time.