Coordinated Universal Time

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The difference between the astronomically determined duration of the day and 86400 SI seconds is also called excess of length of day (LOD).The variations in LOD can be split into several components, according to their causes. The total variation is shown in the upper part of the figure, without oscillations induced by the tides of the solid Earth and oceans, are shown separately for the long and short periods. The dynamical influence of the liquid core of the Earth and climatic variations in the atmosphere account for slow variations (trend in the upper part of the figure). The rest of the atmospheric excitation can be split into a seasonal oscillation and residual oscillation, which includes 50-day oscillations as well as large anomalies like the one associated with the 1983 El Niño event.

Coordinated Universal Time (UTC, French: Temps Universel Coordonné) is a time standard based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the Earth's slowing rotation. Leap seconds are used to allow UTC to closely track UT1, which is mean solar time at the Royal Observatory, Greenwich.

The difference between UTC and UT1 is not allowed to exceed 0.9 seconds, so if high precision is not required the general term Universal Time (UT) may be used.

In casual use, Greenwich Mean Time (GMT) can be considered equivalent to UTC or UT1 when fractions of a second are not important. Owing to the ambiguity as to whether UTC or UT1 is meant, GMT is generally avoided in technical contexts.

Time zones around the world can be expressed as positive or negative offsets from UTC; UTC replaced GMT as the basis for the main reference time scale or civil time in various regions on January 1, 1972.