Mission Elapsed Time
Mission Elapsed Time (MET) 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.
One of the reasons we use MET is to make our premission planning more independent of launch time. As an example, imagine you are taking a trip in your car. You can plan an itinerary as follows:
But, suppose you have a window during which you will leave, for example, between 7:30am and 8:30am. You can then build an itinerary based on Elapsed Time:
Now, if you set the clock in your car to show Elapsed Time, you will know when to stop to walk the dog without having to recalculate your schedule.
There are MET clocks on board; the other on-board clock is the GMT clock.