(GMT) is the time as determined by the rotation of the Earth as measured at the Royal Observatory in for 325 years.
Since most computer and aircraft systems use Greenwich Mean Time as their reference time; the world will have to wait anxiously for the clocks to tick way to this historic moment. Yet as midnight approaches in London this New Year's Eve half the world will already be celebrating the year 2000 unsure if they have beaten the millennium bug.
Starting at noon GMT on 31 December 1999, when the year 2000 arrives at the International Date Line, the parties will begin in the South Pacific and then, hour-by-hour, across the Eastern Hemisphere of the globe.
It will not be until Big Ben chimes midnight that the year 2000 has officially arrived, in Greenwich Mean Time, and the world will know whether the y2k menace has any impact.
Universal Co-ordinated Time (UTC) is atomic time which is a more accurate source of time. It never differs by more than a second difference from GMT. UTC is kept in synch with GMT by inserting leap seconds into UTC. NPL provide atomic time for the United Kingdom.
This clock will countdown to the start of the year 2000. When it reaches zero the world will know if the computers and the aircraft will crash. Since most computer and aircraft systems use Greenwich Mean Time.
Midnight Greenwich Mean Time marks the start of the Universal Day. At midday Greenwich Mean Time for an instant all the world is in the same day by local time.
For a graphic description of how this works check out this (Globe Viewer)