Coordinated Universal Time or UTC, the basis for civil time, differs by an integral number of seconds from atomic time and a fractional number of seconds from UT1. Time zones around the world are expressed as positive and negative offsets from UTC. UTC is the successor of Greenwich Mean Time, abbreviated as GMT, and is still colloquially called GMT sometimes. The new name was coined to eliminate having the name of a specific location in an international standard. UTC bases time measurement on atomic standards rather than GMT's celestial ones.
Because the rotation of the Earth slows down, GMT lags behind atomic time, measured by atomic clocks. UTC is kept within 0.9 s of UT1; leap seconds are added (or, theoretically, subtracted) at the end of a month as necessary. To date all such adjustments have been positive, adding a leap second at 23:59:60. The issuing of leap seconds is determined by the International Earth Rotation Service, based on their measurements of the Earth's rotation.
"UTC" is not a real abbreviation; it is a variant of Universal Time, abbreviated UT, and has a modifier C (for "coordinated") appended to it just like other variants of UT. It may be regarded as a compromise between the English abbreviation "CUT" and the French abbreviation "TUC".
|Eastern Standard Time||UTC -5|
|Central Standard Time||UTC -6|
|Mountain Standard Time||UTC -7|
|Pacific Standard Time||UTC -8|
|Alaska Standard Time||UTC -9|
|Hawaii-Aleutian Standard Time||UTC -10|
International standard UTC time can only be determined to the highest precision after the fact, as atomic time is determined by the reconciliation of the observed differences between an ensemble of atomic clocks maintained by a number of national time bureaus. This is done under the auspices of the International Bureau of Weights and Measures (BIPM). However, local clusters of atomic clocks are sufficient for accuracy to within a few tens of nanoseconds.
UTC presents problems for computer systems such as Unix which store time as the number of seconds from a reference time. Because of leap seconds, it is impossible to determine the representation of a future date, because the number of leap seconds included in that date is unknown.
There are some classes of software UTC clocks:
Wikipedia's own servers use Coordinated Universal Time, but logged-in users can set their time zone in the user preferences, see also Wikipedia:User_preferences_help#Textbox_and_time.