[1] Mission Elapsed Time - Wikipedia, the free encyclopedia ( 版) https://en.wikipedia.org/wiki/Mission_Elapsed_Time

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]

[2] Mission Elapsed Time - Wikipedia, the free encyclopedia ( 版) https://en.wikipedia.org/wiki/Mission_Elapsed_Time

When the shuttle visits ISS the ISS-crew usually adjusts their workday to the MET clock to make work together easier.

[3] Atlas V N22 - Starliner OFT (uncrewed) - 20 Dec 2019 (11:36 UTC) - DISCUSSION () https://forum.nasaspaceflight.com/index.php?topic=43958.860

[4] 打ち上げからの時刻を T+h:m:s のように表記することがしばしばあります。