<time, standard> (UT) The mean solar time along the prime meridian (0 longitude) that runs through the Greenwich Observatory outside of London, UK, where the current system originated. UT is tied to the rotation of the Earth in respect to the fictitious "mean Sun". Greenwich Mean Time (GMT) was measured from Greenwich mean midday until 1925 when the reference point was changed from noon to midnight and the name changed to "Universal Time". There are three separate definitions, UT0, UT1, and UT2, depending on which corrections have been applied to the Earth's motion. Coordinated Universal Time is kept within 0.9 seconds of UT1, by addition of leap seconds to International Atomic Time. (2001-08-02)

<time, standard> (UTC, World Time) The standard time common to every place in the world. UTC is derived from {International Atomic Time} (TAI) by the addition of a whole number of "leap seconds" to synchronise it with Universal Time 1 (UT1), thus allowing for the eccentricity of the Earth's orbit, the rotational axis tilt (23.5 degrees), but still showing the Earth's irregular rotation, on which UT1 is based. Coordinated Universal Time is expressed using a 24-hour clock and uses the Gregorian calendar. It is used in aeroplane and ship navigation, where it also sometimes known by the military name, "Zulu time". "Zulu" in the phonetic alphabet stands for "Z" which stands for longitude zero. UTC was defined by the International Radio Consultative Committee (CCIR), a predecessor of the ITU-T. CCIR Recommendation 460-4, or ITU-T Recommendation X.680 (7/94), contains the full definition. The language-independent international abbreviation, UTC, is neither English nor French. It means both "Coordinated Universal Time" and "Temps Universel Coordonné". {BIPM (http://bipm.fr/enus/5_Scientific/c_time/time_1.html)}. {time/time.html">The Royal Observatory Greenwich (http://rog.nmm.ac.uk/leaflets/time/time.html)}. {History of UTC and GMT (http://ecco.bsee.swin.edu.au/chronos/GMT-explained.html)}. {U.S. National Institute of Standards & Technology (http://its.bldrdoc.gov/fs-1037/dir-009/_1277.htm)}. {UK National Physical Laboratory (http://npl.co.uk/npl/ctm/time_scales.html)}. {US Naval Observatory (http://tycho.usno.navy.mil/systime.html)}. {International Telecommunications Union (http://itu.int/radioclub/rr/arts02.htm)}. {Earth's irregular rotation (http://foldoc.org/pub/misc/earth_rotation)}. (2001-08-30)