Coordinated Universal Time

(UCT, Universal Coordinated Time, Greenwich Mean Time,

GMT, World Time) The standard time common to every place in

the world. UCT nominally reflects the mean solar time along

the prime meridian (0 longitude) that runs through the

Greenwich Observatory outside of London, where the current

system originated. The UCT is based on an atomic clock to

which adjustments in units of a second (called a leap second)

are sometimes made to allow for variations in the solar cycle.

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 as "Zulu

time" [why?].

UCT was defined by the International Radio Consultative

Committee ({CCIR}), a predecessor of the {ITU-T}, and is

maintained by the Bureau International des Poids et Mesures

(BIPM). CCIR Recommendation 460-4, or ITU-T Recommendation

X.680 (7/94), contains the full definition.

(1999-12-09)