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)[1] 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[2], 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 interchangeably, but UTC is clearly defined and based on modern atomic clocks. UTC is also called Zulu time. UTC normally have 86.400 seconds per day, but is kept within 0,9 second of UT1 by introducing an occasional leap second.[3]. UTC is normally used as time reference when accuracy better than one second is not required.