Time standards

Time standards are used to coordinate the time internationally such as GMT, UT, UTC and TAI.

GMT

GMT or Greenwich Mean time originally referred to as the mean solar time clock in Greenwich, England. GMT was internationally adopted in 1884 and has historically used to different terms sometimes numbering hours starting at midnight and sometimes starting at noon. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.

UT

Universal Time (UT) is a time standard based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC. In fact, the expression "Universal Time" is ambiguous, as there are several versions of it, the most commonly used being UTC and UT1

UT1

UT1 is a astronomical time standard measuring the mean rotation of the Earth, computed from observations of distant quasars, satellites and laser ranging of the moon. UT1 is the same everywhere on the Earth.

UTC

The GMT and the UTC time standards are often used