Coordinated Universal Time or UTC is an atomic realization of Universal Time (UT) or Greenwich Mean Time (GMT), the astronomical basis for civil time. Time zones around the world are expressed as positive and negative offsets from UT.
"UTC" is not a true acronym; it is a variant of Universal Time, UT, and has a modifier C (for "coordinated") appended to it just like other variants of UT. It may be regarded as a compromise between the English acronym "CUT" and the French acronym "TUC" (temps universel coordonné). It is sometimes erroneously expanded into "Universal Time Code". Originally, the local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard at the 1884 International Meridian Conference, leading to the widespread use of Greenwich Mean Time in order to set local clocks. This location was chosen because by 1884 two-thirds of all charts and maps already used it as their prime meridian.