This stands for Coordinated Universal Time (the acronym comes from the French Universel Temps Coordonné). It forms the basis of all civil time-keeping and most of the worlds clocks, computers and broadcast radio signals are set either to UTC or a fixed offset from it, according to the local time zone.

UTC is derived directly from atomic clocks which tick at a constant rate because this is the most convenient way to keep time. However, it is also tied to the rotation of the Earth, which defines Universal Time (UT) by the occasional introduction of leap seconds at the end of June or December. This is necessary because the Earth doesn't rotate at a constant rate and is being slowed down gradually by tidal friction, as well as unpredictable short term variations caused by geophysical processes. A second of UTC is always the same length, and is equal to a second of atomic time.

UTC was adopted on January 1st 1972, and replaced Greenwich Mean Time (GMT), although the latter term is still used by many people.

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