Civil clock time is based on "Coordinated Universal Time" (UTC), which is maintained by extremely precise atomic clocks. In contrast, the rotation of the Earth, measured by the UT1 timescale, is irregular; the solar day is gradually but unevenly becoming longer, mainly due to the tidal acceleration of the Moon. In order to keep solar time close to civil time, UTC is corrected by a leap of 1 second. The rotation of the earth is now already a bit slower than it should be in order to have a day of exactly 24 hours. If the rotation would now remain constant, leap seconds would be necessary in regular intervals. Because the earth is continually slowing down, the interval between two leap seconds is becoming smaller over longer timescales. 50000 years in the future one can expect to have more than one leap second every day if the time system is not going to be changed.
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 occurred at the end of a June 30th or December 31st. Unlike leap days, they occur simultaneously worldwide; for example, a leap second on 31 December will be observed as 6:59:60 pm U.S. 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 1972 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. In July 2004, the IERS announced that there will NOT be a leap second at the end of December 2004, so the offset between UTC and TAI will likely remain unchanged at least until July 2005.
It is the responsibility of the International Earth Rotation and Reference Systems Service to measure the Earth's rotation and determine whether a leap second is necessary. Their determination is announced in Bulletin C, published every six months.
Note that leap seconds have nothing to do with leap years.
- IERS Bulletin C, where leap seconds are announced
- IERS information about Bulletin C and when leap seconds may occur
- IERS Archive, to view old announcements
- USNO article on leap seconds