Coordinated Universal Time (UTC) is a high-precision atomic time standard. UTC has uniform seconds defined by International Atomic Time (TAI), with leap seconds announced at irregular intervals to compensate for the Earth's slowing rotation and other discrepancies. Leap seconds allow UTC to closely track Universal Time, a time standard based not on the uniform passage of seconds, but on the Earth's angular rotation.
Time zones around the world are expressed as positive or negative offsets from UTC. Local time is UTC plus the time zone offset for that location, plus an offset (typically +1) for daylight saving time. UTC replaced Greenwich Mean Time on 1 January, 1972 as the basis for the main reference time scale or civil time in various regions.As a time scale, UTC divides up time into days, hours, minutes, and seconds. Each day contains 24 hours and each hour contains 60 minutes, but the number of seconds in a minute is slightly variable.
Most UTC days contain exactly 86,400 seconds, with exactly 60 seconds in each minute. Since the mean solar day is slightly longer than 86,400 SI seconds, occasionally the last minute of a UTC day will have 61 seconds. The extra second is called a leap second. It accounts for the grand total of the extra length (about 2 milliseconds each) of all the mean solar days since the previous leap second. The last minute of a UTC day is allowed to contain 59 seconds to cover the remote possibility of an Earth rotating faster, but that has never happened.Despite the initial controversy, it became clear that basing time signals on atomic clocks was an improvement over the prior system. However, it was widely desired to keep civil time synchronised with the Earth's rotation, and many uses of time signals (such as for navigation)
