Coordinated Universal Time

Resolution 2 of the 26th CGPM (2018) gives the definitions. TAI "is a continuous time scale produced by the BIPM based on the best realizations of the SI second", and UTC is "a time scale produced by the BIPM with the same rate as TAI, but differing from TAI only by an integral number of seconds". The resolution adds that "UTC produced by the BIPM, based on TAI, is the only recommended time scale for international reference and the basis of civil time in most countries". The whole-second offset is what allows an atomic time scale to stay close to the turning Earth.

The Earth is the reason the offset changes. BIPM states that "UTC is obtained from International Atomic Time (TAI) by the insertion of leap seconds", on IERS advice, "to ensure approximate agreement with the time derived from the rotation of the Earth". The 2018 resolution notes that "users can derive the rotation angle of the Earth by applying to UTC the observed or predicted values of UT1 - UTC, as provided by the IERS". The Royal Observatory Greenwich puts the tolerance in numbers: the difference is made "in order to keep UTC within 0.9 seconds of the time determined by the mean position of the Sun (UT1)".

"GMT" is a separate matter. The Royal Observatory records that "From 1884 until 1972, GMT was the international standard of civil time. Though it has now been replaced by Coordinated Universal Time (UTC)", UTC being "the modern equivalent of Greenwich Mean Time (GMT), which is never more than half a second different from it". On a watch dial, by contrast, "GMT" is a trade label for a second time-zone display: it says nothing about which time scale the watch is set to, and nothing about UTC. This Codex's entry on the dual-time watch covers that function.