DAVID G. SIMPSON

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LEAP SECONDS

A leap second is an extra second inserted at the end of a day in order to set the Coordinated Universal Time (UTC) clock back one second. This is done to keep UTC in step with the UT1 time scale, which is based on the Earth's rotation and runs slightly slower than atomic clocks.

Leap seconds occur at irregular intervals. One is generally added at the end of the day on a June 30 or December 31, although it is possible for them to occur on March 31 or September 30, if necessary. When a leap second occurs, the second following 23:59:59 UTC is called 23:59:60 UTC, which is followed by 00:00:00 UTC of the next day. Negative leap seconds are also possible, but have never occurred.

This table shows the dates of all leap seconds that have occurred to date. Here TAI is International Atomic Time, and UTC is Coordinated Universal Time.

 Date  TAI-UTC 
 Dec. 31, 1971 
10
 June 30, 1972 
11
 Dec. 31, 1972 
12
 Dec. 31, 1973 
13
 Dec. 31, 1974 
14
 Dec. 31, 1975 
15
 Dec. 31, 1976 
16
 Dec. 31, 1977 
17
 Dec. 31, 1978 
18
 Dec. 31, 1979 
19
 June 30, 1981 
20
 June 30, 1982 
21
 June 30, 1983 
22
 June 30, 1985 
23
 Dec. 31, 1987 
24
 Dec. 31, 1989 
25
 Dec. 31, 1990 
26
 June 30, 1992 
27
 June 30, 1993 
28
 June 30, 1994 
29
 Dec. 31, 1995 
30
 June 30, 1997 
31
 Dec. 31, 1998 
32
 Dec. 31, 2005 
33
 Dec. 31, 2008 
34

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