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Did you know that 2005 is going to be a long year? It certainly seems so with us; the arrival of Sam in July has made it feel like we're getting value for money. But this year is 1 second longer than most other years, thanks to the introduction of a Leap Second. At midnight, New Year's Eve, the time will roll from 23:59:59 to 23:59:60 before going to 00:00:00. (A good tidbit of knowledge for a pub quiz if ever there was one.)
So what's the reason for all this? Well, a second is historically based on the rotation of the Earth, but this hasn't been a constant speed through eternity. So the SI Second (International Standard in French) is defined as a number of vibrations of an atom like Caesium, which give us atomic clocks, which have been a constant for a long time (though back in the picoseconds after the big bang, it's not so certain). Since the Earth is slowing down gradually, the time as calculated from atomic clocks has been drifting apart from 'Earth time'.
To fix this problem, a Leap Second is added occasionally to re-align Earth Time with Atomic Time, and is known as UTC (Universal Co-ordinated Time, from the French). Essentially, the difference between GMT and UTC is that the latter has leap seconds, whereas the former does not; but otherwise, they're the same.
Atomic clocks in GPS use GMT/Atomic time for their calculations, and as such are drifting apart (currently about 14 seconds adrift). However, that doesn't make for much difference, since the GPS receivers download updates to their almanack periodically to take into account of variations such as these, and most of them will show time in UTC instead of GMT.
Oh, and as for the leap second ... it's applied simultaneously around the world. So the second is only added at midnight in Zulu time (GMT+0:00); in America's East Coast, for example, the Leap Second will be added between 18:59:59->18:59:60->19:00:00. Time zones ahead of GMT (Alpha through Mike time) get the Leap Second added on the 1st January 2006. If you've got a computer synchronized with a time server such as NTP, then your computer will slowly apply the difference between the two to synchronise the clocks with UTC. (You should be able to see the offset via a ntpdate command jump from a near-zero value to a near-one value around this time, and then gradually reduce:
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"My dreams are all dead & buried. Sometimes I wish the sun would just explode. When God comes and calls me to his kingdom, I'll take all you sonsabitches when I go"
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