Why Are There Two Different Countdown Clocks?
Time Until Launch:
simple countdown clock This clock is a simple, real-time countdown to the scheduled launch time. It will be reset as soon as possible in the event of a delay. After launch, the clock will run forward, recording mission elapsed time (MET).

Official Countdown Clock:
official countdown clock This is a virtual replica of the official countdown clock at the launch site at NASA's Kennedy Space Center. It begins T-43 hours and counting mark before launch and mirrors the real clock by pausing during the various built-in holds during the countdown. For an explanation of the built-in holds, read "Countdown 101".