Mission Elapsed Time (MET) is used by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

during their space missions, most notably during their

Space Shuttle The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...

missions. Due to the fact that timing is very consequential in a mission, all events after launch are scheduled on the Mission Elapsed Time. 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. MET was formerly called Ground Elapsed Time (GET) prior to the Space Shuttle. The

International Space Station The International Space Station (ISS) is a large space station that was Assembly of the International Space Station, assembled and is maintained in low Earth orbit by a collaboration of five space agencies and their contractors: NASA (United ...

(ISS) does not use an MET clock since it is a "permanent" and international mission. The ISS observes

Greenwich Mean Time Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory, Greenwich, Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being ...

(UTC/GMT). The shuttles also had UTC clocks so that the astronauts could easily figure out what the "official" time aboard ISS was. In 2019, a test flight of the Boeing CST-100 Starliner spacecraft suffered a mission anomaly through an incorrectly set Mission Elapsed Time on the vehicle.

References

Spaceflight Time scales {{space-stub