NASA run
two concurrent countdown clocks. The actual time to launch (presently showing around 8 hours but likely to change after the latest hitch), includes several planned hold points where they stop the other clock - the one announced on the NASA TV commentary.
The Countdown 101 page gives more info:
http://www.nasa.gov/mission_pages/la...ntdown101.html
Today's launch (tomorrow morning our time), has just been scrubbed - they're draining the LOX tank now, then the hydrogen one. Grrr!