And later, I found a reference which further answered my own question - Seago,
John H.; Seidelmann, P. Kenneth, The mean-solar-time origin of Universal Time
and UTC - I see this conclusion:
“[...] UT1 still appears synonymous with "mean solar time at the prime
meridian" to within a fraction of a second, despite the definition and
maintenance of UT1 having evolved significantly over the past century. The
"prime meridian" in this case implies the International Reference Meridian that
includes the terrestrial origin from which Universal Time is measured, and
which has advanced approximately 0.36 [seconds] to the east of the
internationally recommended origin of longitude -- the Airy transit instrument
at Greenwich. This translation resulted from changing conventions and methods
by which the relationship between Universal Time and the terrestrial reference
were maintained over time by the Bureau International de l'Heure.”
So, what I take from this is, 1) UT1 (and hence UTC) is based on the
International Reference Meridian, not the Prime Meridian (Greenwich). 2)
“True” GMT (if the mean sun can accurately be measured and predicted by
formulae, which has its own set of problems and error bars) lags UT1 by about
0.36 seconds; but it doesn’t really matter, since the whole world is now based
on the IRM.
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