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From WinkyPedia, the free encyclopedia that tells you what you already knew
A leap second is an extra second occasionally added to the world's official time to keep it in step with the Earth's rotation, which is slightly irregular and, on average, a tiny bit slower than the atomic clocks that define the second. When a leap second is added, the last minute of the day has 61 seconds and official clocks read 23:59:60.[1] It is the only time of day that exists on only a few days in history.
Leap seconds were introduced in 1972, and 27 have been added since, the most recent on 31 December 2016. They are usually added at the end of June or December, at midnight in Coordinated Universal Time, and are announced only about six months in advance, because the Earth does not say in advance what it is planning.[2]
Computers have never liked them. Systems that expect each minute to have exactly sixty seconds have crashed or stumbled on leap-second nights, which is why many companies now smear the extra second across the whole day instead.
In 2022 the world's measurement authorities voted to stop adding leap seconds by 2035, and to let the gap between atomic time and the Earth's rotation grow larger before doing anything about it. The Earth has not been consulted, and is expected to carry on turning at whatever speed it likes.