World map of current time zones
Coordinated Universal Time (
French: Temps universel coordonné), abbreviated to UTC, is the primary
time standard by which the world regulates clocks and time. It is within about 1 second of
mean solar time at
0° longitude; it does not observe
daylight saving time. For most purposes, UTC is considered interchangeable with
Greenwich Mean Time (GMT), but GMT is no longer precisely defined by the scientific community.
The first Coordinated Universal Time was informally adopted on 1 January 1960.
The system was adjusted several times, including a brief period where time coordination radio signals broadcast both UTC and "Stepped Atomic Time (SAT)" until a new UTC was adopted in 1970 and implemented in 1972. This change also adopted
leap seconds to simplify future adjustments. 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."
A number of proposals have been made to replace UTC with a new system that would eliminate leap seconds, and the decision to remove them altogether has been tabled until 2023.
The current version of UTC is defined by
International Telecommunications 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 slowing of
Earth's rotation. Leap seconds are inserted as necessary to keep UTC within 0.9 seconds of
universal time, UT1.
 See the "
Current number of leap seconds" section for the number of leap seconds inserted to date.