Greenwich Mean Time (GMT) is the earlier name for Coordinated Universal Time (UTC). It is the mean solar time measured along the prime meridian (which passes through Greenwich, England at zero degrees longitude), and provides a common reference time for all other time zones on the Earth. This simplifies the task of identifying when particular celestial events will occur.

Zero hours UTC occurs at midnight, when the Sun undergoes a lower meridian transit of the prime meridian.

UTC is also known as Universal Time (UT).