It still seems to be common among groups outside the computing and scientific worlds to talk about “GMT” rather than “UTC”. The trouble with “GMT” is that it doesn’t seem to be clearly defined; it might or might not differ from UTC by up to 0.9 seconds. The (unadjusted) time reference defined by an official collection of atomic clocks worldwide is called “TAI”. This pays no heed to the slowing rotation of the Earth. Astronomers prefer a time reference called “UT1”, which is based on the actual rotation of the Earth (subject to some time-averaging, I think). The definition of “UTC” comes from TAI, adjusted by applying leap seconds. These accumulate as needed so as to keep UTC within 0.9 seconds of UT1, and we are currently up to 36 of them. That is, at 0:00:00 UTC today, TAI was actually 0:00:36. The trouble with GMT is that it is not officially clear whether it is the same as UT1, or the same as UTC. Given that it was originally defined at the Greenwich Observatory, which was (is?) run by astronomers, one would think they would make it the same as UT1. But the practical fact is, our usual everyday timekeeping is based on standard time zones, which are all defined as whole-second (indeed, whole multiples of a half-hour) offsets from UTC.