text: why does the clock get set back 1 second every few years once?   leap seconds civil time is occasionally adjusted by one second increments to ensure that the difference between a uniform time scale 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.  historically, the second was defined in terms of the rotation of the earth as 1/86,400 of a mean solar day.  in 1956, the international committee for weights and measures, under the authority given it by the tenth general conference on weights and measures in 1954, defined the second in terms of the period of revolution of the earth around the sun for a particular epoch, because by then it had become recognized that the earth's rotation was not sufficiently uniform as a standard of time.  the earth's motion was described in newcomb's tables of the sun, which provides a formula for the motion of the sun at the epoch 1900 based on astronomical observations made during the eighteenth and nineteenth centuries.  the ephemeris second thus defined is   for more click the source link.