The master timing unit is a stable crystal-controlled timing source for the orbiter. It sends serial time reference signals to the onboard computers, PCMMUs, payloads and various time display panels. It also provides synchronization for instrumentation payloads and other systems. It includes separate time accumulators for Greenwich Mean Time and mission elapsed time, which can be reset or updated from the ground via uplink through the onboard computer or by the flight crew through the use of the flight deck display and control panel keyboard and CRT time displays.

The signal flows from the 4.608-MHz oscillators to the output of the GMT and MET accumulators. GMT or MET can be displayed on the mission time displays on panels O3 and A4 by positioning the mission timer switch on the respective panel to GMT or MET . In addition, event time displays are located on panels F7 and A4. Separate time accumulators used for the GMT and MET clocks accumulate time in days, hours, minutes, seconds and milliseconds. The GMT capacity is 366 days, 23 hours, 59 minutes, 59 seconds and 999.875 milliseconds. For MET, the capacity is 365 days, 23 hours, 59 minutes, 59 seconds and 999.875 milliseconds. Before flight, both time displays can be updated and reset by ground equipment or by the flight crew using onboard controls. During flight, the GMT and MET accumulators are updated at a predetermined time by uplink and the onboard computer or by voice command. They are entered through the flight deck display and control panel keyboard and CRT display.