<P> Because of time dilation, a standard clock not on the geoid, or in rapid motion, will not maintain synchronicity with UTC . Therefore, telemetry from clocks with a known relation to the geoid is used to provide UTC when required, on locations such as those of spacecraft . </P> <P> It is not possible to compute the exact time interval elapsed between two UTC timestamps without consulting a table that describes how many leap seconds occurred during that interval . By extension, it is not possible to compute the duration of a time interval that ends in the future and may encompass an unknown number of leap seconds (for example, the number of TAI seconds between "now" and 2099 - 12 - 31 23: 59: 59). Therefore, many scientific applications that require precise measurement of long (multi-year) intervals use TAI instead . TAI is also commonly used by systems that cannot handle leap seconds . GPS time always remains exactly 19 seconds behind TAI (neither system is affected by the leap seconds introduced in UTC). </P> <P> For most common and legal - trade purposes, the fractional second difference between UTC and UT (GMT) is inconsequentially small . Greenwich Mean Time is the legal standard in Britain during the winter, and this notation is familiar to and used by the population . </P> <P> Time zones are usually defined as differing from UTC by an integer number of hours, although the laws of each jurisdiction would have to be consulted if sub-second accuracy was required . Several jurisdictions have established time zones that differ by an integer number of half - hours or quarter - hours from UT1 or UTC . </P>

What is the time difference between utc and gmt