The unit of time, the second, was at one time considered to be the fraction
Nowadays the second is defined as the duration of 9 192 631 770
periods of the radiation corresponding to the transition between the two
hyperfine levels of the ground state of the caesium133 atom.
Time is calculated for the world by the BIPM, based on data collected from
the world's most accurate clocks, which are developed by national
laboratories to realize the SI second at the highest level. Collectively
these clocks allow the construction of the international reference time scale (UTC), and individually they also provide essential tools for studies of fundamental physics.
Greenwich Mean Time (GMT) is often interchanged or confused with
Coordinated Universal Time (UTC). But GMT is a time zone and UTC is a
time standard.
Universal Time (UT) is also a time standard based on Earth's rotation. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the Prime Meridian at Greenwich, London, UK.
The time correction DUT1 (sometimes also written DUT) is the difference between Universal Time (UT1), which is defined by Earth's rotation, and Coordinated Universal Time (UTC), which is defined by a network of precision atomic clocks.
UTC is maintained via leap seconds,
such that DUT1 remains within the range −0.9 s < DUT1 < +0.9 s.
The reason for this correction is partly that the rate of rotation of
the Earth is not constant, due to tidal braking and the redistribution of mass within the Earth, including its oceans and atmosphere, and partly because the SI second (as now used for UTC) was already, when adopted, a little shorter than the current value of the second of mean solar time.
Since this system of correction was implemented in 1972, 26 leap seconds have been inserted, the most recent on June 30, 2015 at 23:59:60 UTC. The irregularity and unpredictability of UTC leap seconds is problematic for several areas, especially computing
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