A replacement adopted in 1972 for Universal Time, maintained to scientific standards by atomic clocks. UTC adds or subtracts leap seconds twice a year to adjust for irregularities in the Earth's rotation. UTC is used by NASA's deep-space network for events recorded by when transmission of the event is received (see earth-received time). UTC clocks are also installed on board spacecraft, including space shuttles. However, the times of most events during a flight are measured by mission elapsed time (MET) clocks.