This avoids the constant rescheduling of events in case the launchtime slips.
The MET-clock is set to zero at the moment of liftoff and counts forward in normal days, hours, minutes, and seconds.
For example, 2:03:45:18 MET means it has been 2 days, 3 hours, 45 minutes, and 18 seconds since liftoff.
[1][2] MET was formerly called Ground Elapsed Time (GET) prior to the Space Shuttle.
The shuttles also had UTC clocks so that the astronauts could easily figure out what the "official" time aboard ISS was.