Jaap van Ganswijk wrote in part:
Yes, but I always wonder: Isn't the daylight
time valid in Greenwich itself?
DST is used in Greenwich as in the rest of the EU.
But Greenwich Mean Time is another matter entirely.
GMT is not defined by our modern Time Zone standards,
nor by the day to day position of Sun in the sky
(which was how time was kept throughout most of human history),
but by the longitude where Greenwich is located.
The yearly average (mean) of Sun's position there determines GMT;
Paris Mean Time is different since Paris has a different longitude.
OTOH, their Standard Time is the same, since they're in the same Time Zone.
What's more, GMT is now deprecated in favour of UTC,
Coordinated Universal Time (the initialism shows you that
it's the fault of the French that Greenwich is now ignored),
which is based on the oscillation of caesium ions,
with leap seconds added from time to time to keep the hour
in line with the sunlight, more or less.
-- Toby Bartels