This number has to be so large that it can encompass all the time passed since midnight January 1st, 1970 but sufficiently small that it can fit into existing data structures and keep going enough time in the future. In Android you tell an alarm when to come up by passing a simple number. The 'current millis' story started with me debugging my Android application. More importantly, this site offers a time navigation service for human users and a time authority service for programmatic usage.

You can also convert milliseconds to date & time and the other way around. This site provides the current time in milliseconds elapsed since the UNIX epoch (Jan 1, 1970) as well as in other common formats including local / UTC time comparisons. From this point of view the name “GMT” seems deprecated, but kept around for backward compatibility, traditional timezone based representation of time and sometimes legal reasons. If you were to calculate true GMT today i would see it based on its original definition of 1 second = 1/86400 days and this would for sure return a different absolute value than what UTC gives us. These 2 turning points (different definition of a second and the introduction of leap seconds) ‘forced’ GMT to be the same as UTC based on what seemed a gradual, tacit convention. In 1972 leap seconds were introduced to synchronize UTC time with solar time. UTC’s second is far more precise than GMT's original second. Unlike GMT which is based on solar time and originally calculated a second as a fraction of the time it takes for the Earth to make a full rotation around its axis, UTC calculates a second as “the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom”.

UTC essentially appeared in 1960, GMT being the ‘main thing’ until then. Literature and history are a bit ambiguous.

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