A
leap second is a positive or negative one-
second adjustment to the
Coordinated Universal Time (UTC)
time scale that keeps it close to
mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for
civil time, is maintained using extremely precise
atomic clocks. To keep the UTC time scale close to mean solar time, UTC is occasionally corrected by an intercalary adjustment, or "leap", of one second. Over long time periods, leap seconds must be added at an ever increasing rate (see
ΔT). The timing of leap seconds is now determined by the
International Earth Rotation and Reference Systems Service (IERS). Leap seconds were determined by the
Bureau International de l'Heure (BIH) prior to January 1, 1988, when the IERS assumed that responsibility.
When a positive leap second is added at 23:59:60 UTC, it delays the start of the following UTC day (at 00:00:00 UTC) by one second, effectively delaying the UTC clock. Negative leap seconds have never been needed. For one to be needed within a year the length of day (LOD) would have to decrease by 1⁄365 second, or about three milliseconds. While LOD does fluctuate, it is currently much as it was in 1820, and historic eclipse observations show that LOD tends to increase by roughly two milliseconds per century. A decrease of 4 milliseconds did occur around 1920, but that required more than a decade of continuous acceleration.
http://en.wikipedia.org/wiki/Leap_second#cite_note-0
