The basis for civil timekeeping. It is formally defined by a mathematical formula which relates UT to Greenwich mean sidereal time. Depending on the context, universal time and UT are commonly used to mean 1) UT0, which is dependent on the observer's location; 2) UT1, which removes the effect of the motion of the geographic pole; or 3) coordinated universal time (UTC). Since 1 January 1972, weather services have used UTC as the standard of time. See Also zone time.