Saturday, June 25th, 2016
GMT stands for Greenwich Mean Time. GMT time is the current time measured on the Prime Meridian (0 degrees longitude). The Prime Meridian runs through the Greenwich Observatory located in the suburbs of London. It was developed around 1850 by the British Empire and internationally adopted in the International Meridian Conference of 1884, in Washington.
GMT is based on the Earth’s rotation (which is irregular) so it actually follows a fictitious Mean Sun that moves at a uniform speed along the equator. GMT was at first calculated by the 24-hour clock starting at noon. However, since 1925, the day of GMT starts at midnight.
Over time, time zones became established based on GMT as being x number of hours ahead or behind GMT.
GMT was later renamed to Universal Time, or UT. It is also known as UT0. It becomes UT1 when it is corrected for the irregular movements of the terrestrial poles, also known as the Chandler wobble. The Earth’s poles do not spin perfectly in a straight line.
GMT was replaced in 1972 by the Coordinated Universal Time (UTC).
UTC, which stands for Coordinated Universal Time in English and Temps Universel Coordonné in French, was abbreviated UTC as a compromise between CUT and TUC in English and French, respectively.
UTC, while based on zero degrees longitude, which passes through the Greenwich Observatory, is based on atomic time and includes leap seconds as they are added to our clock every so often.
The difference between UTC and UT1 cannot exceed 0.9 s, so if high precision is not required the general term Universal Time (UT) (without a suffix) may be used.
Summing-up: In casual use, Greenwich Mean Time (GMT) is the same as UTC and UT1. Owing to the ambiguity of whether UTC or UT1 is meant, and because timekeeping laws usually refer to UTC, GMT is avoided in careful writing. Today, it is most appropriate to use and refer to time based on UTC and not on GMT.