A leap second is adding one extra second to time sometimes. This is done to fix the small difference between very exact time (kept by atomic clocks) and the less exact solar time (based on the Earth’s rotation). The time system we use (UTC) would get too far ahead of solar time without these fixes. Leap seconds were first used in 1972. Since then, 27 leap seconds have been added, the last one on December 31, 2016.^[1] All of them added one second. Taking away a second is possible, but it has never happened.

The Earth's rotation speed changes because of weather and natural events.^[2] This makes leap seconds hard to predict. A group called the International Earth Rotation and Reference Systems Service (IERS) decides when to add them, usually about six months before, so the difference between UTC and solar time never gets bigger than 0.9 seconds.^[3][4]

Leap seconds cause problems, especially for systems that need very exact timing. Some computers do not update correctly, so their time can be wrong.^[5] After many years of talks, it was decided in November 2022 that leap seconds will be stopped by or before 2035.^[6][7]