The leap seconds cannot be predicted far in advance due to the unpredictable rate of the rotation of Earth. Decisions to introduce a leap second are announced at least six months in advance in "Bulletin C" produced by the International Earth Rotation and Reference Systems Service. Thus, in the UTC time scale, the second and all smaller time units (millisecond, microsecond, etc.) are of constant duration, but the minute and all larger time units (hour, day, week, etc.) are of variable duration. The number of seconds in a minute is usually 60, but with an occasional leap second, it may be 61 or 59 instead. Each day contains 24 hours and each hour contains 60 minutes. Days are conventionally identified using the Gregorian calendar, but Julian day numbers can also be used. UTC divides time into days, hours, minutes and seconds. The International Space Station also uses UTC as a time standard.Īmateur radio operators often schedule their radio contacts in UTC, because transmissions on some frequencies can be picked up in many time zones. Weather forecasts and maps all use UTC to avoid confusion about time zones and daylight saving time. for flight plans and air traffic control. UTC is also the time standard used in aviation, e.g. For sub-microsecond precision, clients can obtain the time from satellite signals.
If only milliseconds precision is needed, clients can obtain the current UTC from a number of official internet UTC servers. The Network Time Protocol (NTP), designed to synchronise the clocks of computers over the Internet, transmits time information from the UTC system. UTC is used in many Internet and World Wide Web standards. In 1995, the island nation of Kiribati moved those of its atolls in the Line Islands from UTC−10 to UTC+14 so that Kiribati would all be on the same day. The westernmost time zone uses UTC−12, being twelve hours behind UTC the easternmost time zone uses UTC+14, being fourteen hours ahead of UTC. Time zones around the world are expressed using positive or negative offsets from UTC, as in the list of time zones by UTC offset. The compromise that emerged was UTC, which conforms to the pattern for the abbreviations of the variants of Universal Time (UT0, UT1, UT2, UT1R, etc.). English speakers originally proposed CUT (for "coordinated universal time"), while French speakers proposed TUC (for " temps universel coordonné"). This abbreviation comes as a result of the International Telecommunication Union and the International Astronomical Union wanting to use the same abbreviation in all languages. The official abbreviation for Coordinated Universal Time is UTC. See the " Current number of leap seconds" section for the number of leap seconds inserted to date. Leap seconds are inserted as necessary to keep UTC within 0.9 seconds of the UT1 variant of universal time. The current version of UTC is defined by International Telecommunication Union Recommendation (ITU-R TF.460-6), Standard-frequency and time-signal emissions, and is based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the accumulated difference between TAI and time measured by Earth's rotation. A decision whether to remove them altogether has been deferred until 2023.
This CCIR Recommendation 460 "stated that (a) carrier frequencies and time intervals should be maintained constant and should correspond to the definition of the SI second (b) step adjustments, when necessary, should be exactly 1 s to maintain approximate agreement with Universal Time (UT) and (c) standard signals should contain information on the difference between UTC and UT." Ī number of proposals have been made to replace UTC with a new system that would eliminate leap seconds. This change also adopted leap seconds to simplify future adjustments. The system has been adjusted several times, including a brief period during which the time-coordination radio signals broadcast both UTC and "Stepped Atomic Time (SAT)" before a new UTC was adopted in 1970 and implemented in 1972. UTC was first officially adopted as CCIR Recommendation 374, Standard-Frequency and Time-Signal Emissions, in 1963, but the official abbreviation of UTC and the official English name of Coordinated Universal Time (along with the French equivalent) were not adopted until 1967. The coordination of time and frequency transmissions around the world began on 1 January 1960. It is effectively a successor to Greenwich Mean Time (GMT). It is within about 1 second of mean solar time (such as UT1) at 0° longitude (at the IERS Reference Meridian as the currently used prime meridian) and is not adjusted for daylight saving time. Coordinated Universal Time or UTC is the primary time standard by which the world regulates clocks and time.
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