The world's official timekeeper, International Earth Rotation and Reference Systems Service (IERS) in Paris, has mandated that 1 extra second be added to 31 December 2008 so as to reconcile 2 different time scales, one being the atomic time pieces and the other which is bbased on Earth's rotation on its own axis. This extra second is known as leap second, unlike the leap year it is not added every once it 4 years, it is only on a case by case basis, this year will be the 24th time the leap second is added since the practice started in 1972.
The IERS was established in its present form in 1987 by the International Astronomical Union and the International Union of Geodesy and Geophysics, replacing the earlier International Polar Motion Service (IPMS) and the earth-rotation section of the Bureau International de l'Heure (BIH). Among its other functions, the IERS is responsible for announcing leap seconds.
Leap seconds are necessary because time is measured using stable atomic clocks (TAI or International Atomic Time), whereas the rotation of Earth slows down continually, though at a slightly variable rate.
Leap seconds were added in the following days:
1972: 30th June & 31st December
1973, 1974, 1974, 1976, 1977, 1978, 1979, 1987, 1989, 1990, 1995, 1998, 2005, 2008: 31st December
1981, 1982, 1983, 1985, 1992, 1993, 1994, 1997: 30th June
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