The principal astronomical cycles are the day (based on the rotation of the Earth on its axis),
the year (based on the revolution of the Earth around the Sun), and the month (based on the
revolution of the Moon around the Earth). The week is a non-astronomical cycle.
The complexity of calendars arises because these cycles of revolution do not comprise an integral number of days, and because astronomical cycles are neither constant nor perfectly commensurable with each other.
One second is 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 cesium 133 atom.
Coordinated Universal Time (UTC) is based on the time on the 0 degree longitude passing through Greenwich, England.
"The logic behind Coordinated Universal Time is that: there are 7 Universal Times (all within 1 second of each other), and UTC is the "coordinated version of 'Universal Time'", hence the word order of Coordinated Universal Time. The abbreviation UTC is a language-independent international abbreviation, it is neither English nor French. It means both 'Coordinated Universal Time' and 'Temps Universal Coordonné'.
UTC isn't the same as GMT, as UTC is an ATOMIC time-scale, while GMT (strictly speaking UT1 [UT-one]) is tied to the rotation of the Earth in respect to the fictitious 'mean Sun'. UTC is, however, kept within 0.9 seconds of UT1, by virtue of leap seconds.
You can also listen to NIST's short-wave Radio Station WWV on 2.5, 5, 10, 15 or 20MHz; or phone +1-303-499-7111 (a toll call to Boulder, Colorado) for accurate time of day and CORRECT word order.
Time is also available from Canadian Radio Station CHU on 3.33, 7.335 or 14.67 MHz (in both English and French)."
Many thanks for this text to: Howard Barnes firstname.lastname@example.org
Local time in Greenwich is also called Greenwich Mean Time (GMT), established in 1840 for England, Scotland, and Wales.
Greenwich Mean Time is a widely used historical term, but one that has been used in several ways. Because of the ambiguity, its use is no longer recommended in technical contexts.
(US Naval Observatory)