civil time, local time based on universal time. Civil time may be formally defined as mean solar time plus 12 hr; the civil day begins at midnight, while the mean solar day begins at noon. Civil time is occasionally adjusted by one-second increments to ensure that the difference between a uniform timescale defined by atomic clocks does not differ from the earth's rotational time by more than 0.9 seconds. Coordinated universal time (UTC), an atomic time, is the basis for civil time. Civil time is usually not used, since it depends on the observer's longitude; instead, standard time, which is the same throughout a given time zone, is generally adopted.