The Environment of Space Chapter 2. It is based upon carefully maintained atomic clocks and is kept accurate to within microseconds worldwide. The addition or subtraction of leap seconds, as necessary, at two opportunities every year adjusts UTC for irregularities in Earth's rotation. It is common to see outdated references to GMT, even in currently operating flight projects.
Many locations adjust for standard time or daylight-savings time.
Local time is also determined on other planets when needed. Local time on another planet is conceived as the equivalent time for the distance of the sun from the meridian, corresponding to the same local time on Earth. TRM , Transmission time is the UTC time of uplink from Earth. Knowledge of OWLT is maintained to an accuracy of milliseconds. OWLT varies continuously as the spacecraft's distance from the Earth changes.