UTC replaced everyday GMT starting in the 1960s
Coordinated Universal Time is the world's main civil time standard, successor to Greenwich Mean Time in common use. It rides on International Atomic Time—a weighted average of hundreds of atomic clocks—then inserts leap seconds when Earth rotation drifts. Coordination of broadcasts began 1 January 1960; UTC became an official standard in 1963.
The abbreviation is a diplomatic compromise. The telecommunications and astronomy bodies wanted one label for all languages, so in 1967 the radio committee CCIR adopted the English and French names Coordinated Universal Time and Temps Universel Coordonné with the single acronym UTC, matching the pattern of other Universal Time variants such as UT1. The IAU had coined Universal Time in 1928 as a name for GMT counted from midnight, and until the 1950s time signals followed Earth's rotation. The International Telecommunication Union now defines UTC, and the BIPM calculates it from more than 70 timing centres, publishing their offsets monthly in Circular T.
Because the planet's spin is slightly slower than 86,400 atomic seconds a day, UTC has added leap seconds since 1972, making the day's final minute 61 seconds long. UTC equals atomic time minus the leap seconds accumulated so far, which means nobody can compute the exact number of seconds between two dates without a table, or at all for dates in the future. The disruption these jumps cause led the General Conference on Weights and Measures to vote in 2022 to drop leap seconds by 2035. UTC is only known exactly after the fact; labs broadcast real-time approximations labelled UTC(k).
Time zones range from UTC−12 to UTC+14. In 1995 Kiribati shifted its Line Islands from UTC−10 to UTC+14 so the whole country shared one date. Some zones sit a half or quarter hour off. UTC never changes for summer; the US east coast is five hours behind in winter but four during daylight saving. A device without a UTC setting can use Accra or Reykjavík, which stay on it year-round, unlike London.
Aviators call it Zulu time: Greenwich was the origin of lettered nautical zones and was marked Z, and the NATO word for Z is Zulu. Flight plans, weather maps, the International Space Station and amateur radio schedules all use it, and the Network Time Protocol synchronises computers to it over the internet.
Source: Coordinated Universal Time
