UTC

Coordinated Universal Time, or "UTC", is the reference time zone from which all other time zones around the world are calculated. This was formerly known as Greenwich Mean Time and 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, and because of confusion with English local time (which is the same as UTC during the winter months, but is one hour ahead of UTC in the summer since the introduction of [British Summer Time]?).

UTC's reference point is [Greenwich Observatory]? in Greenwich?, London, England. Theoretically, noon Greenwich Mean Time, now formally known as UT1, coincides with the sun reaching its highest point in the sky in the meridian? at this location (which is also 0 degrees geographic longitude). Because of the Earth's uneven speed in its elliptic orbit, this event may be up to 16 minutes off (known as the analemma?); but this averages out over the year. So UT1 follows a fictituous "mean sun" that moves at uniform speed along the equator in a year, and that appears to move across the sky in a day as the Earth rotates around its axis.

However, UTC is not kept by watching the Sun, but is measured by atomic clocks. Because the rotation of the Earth slows down, it lags behind this atomic time. In order to keep the clock time UTC synchronized to the day and night of UT1, leap seconds are added (or removed) at either the end of June or December whenever necessary. This is announced by the International Earth Rotation Service at http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.dat .

"UTC" does not actually stand for anything; it was chosen as a compromise between the English abbreviation "CUT" and the French abbreviation "TUC".

See http://aa.usno.navy.mil/faq/docs/UT.html, List of countries with their time zone.