Leap second: Difference between revisions

Revision as of 07:22, 7 November 2003

Revision as of 22:35, 22 November 2003

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The instruction to insert a leap second will be given whenever the difference between UTC and UT1 is expected to exceed 0.9 s. After UTC 23:59:59, an additional second at 23:59:60 is counted, before the clock jumps to 00:00:00 of the next day. Negative leap seconds are also possible if the Earth's rotation becomes slightly faster, but this has never happened. In that case, 23:59:58 would be followed by 00:00:00.

Leap seconds can occur only at the end of a month, and have only ever occuredoccurred at the end of a [[30 June]] or [[31 December]]. Unlike [[leap day]]s, they occur simultaneously worldwide; for example, a leap second on 31 December will be observed as 7:59:60 pm [[CET#UTC-5_(EST_-_Eastern_Standard_Time)|US Eastern Standard Time]].

Historically, leap seconds have been inserted about every 18 months. However, as the slowing of the Earth is irregular, it is not possible to predict more than a relatively short time in advance whether a leap second will have become necessary. Between January [[1970]] and November [[2001]], the IERS gave instructions to insert a leap second on 22 occasions. The most recent leap second was 1998-12-31 23:59:60 UTC; the interval since then has been the longest period without a leap second.