<P> Between 1967 and 1971, UTC was periodically adjusted by fractional amounts of a second in order to adjust and refine for variations in mean solar time, with which it is aligned . After 1 January 1972, UTC time has been defined as being offset from atomic time by a whole number of seconds, changing only when a leap second is added to keep radio - controlled clocks synchronized with the rotation of the Earth . </P> <P> The Global Positioning System also broadcasts a very precise time signal worldwide, along with instructions for converting GPS time to UTC . GPS - time is based on, and regularly synchronized with or from, UTC - time . </P> <P> Earth is split up into a number of time zones . Most time zones are exactly one hour apart, and by convention compute their local time as an offset from GMT . For example, time zones at sea are based on GMT . In many locations (but not at sea) these offsets vary twice yearly due to daylight saving time transitions . </P> <P> These conversions are accurate at the millisecond level for time systems involving earth rotation (UT1 & TT). Conversions between atomic time systems (TAI, GPS, and UTC) are accurate at the microsecond level . </P>

Which method of measuring time using time zones