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.
When the shuttle visits ISS the ISS-crew usually adjusts their workday to the MET clock to make work together easier.
 Atlas V N22 - Starliner OFT (uncrewed) - 20 Dec 2019 (11:36 UTC) - DISCUSSION
(2019-12-26 14:44:29 +09:00)