Under an EU directive of 2002, which states that ‘Summer Time’ will be observed between the last Sundays in March and October, clocks in UK revert to GMT (~UTC) on 31 October; changes either way occur at 0100 UTC.
GMT, based on the angular position of the Earth around its axis, and Coordinated Universal Time (UTC), based on over 400 atomic clocks with leap seconds added, should never differ by more than 0.9 seconds.
So UK reverts to its ‘natural’ time, centred on the Greenwich meridian and in the 15º longitude NATO time-zone ‘Zulu’ (Z), used in international travel, timing - and astro calculations. The 24 lettered time-zones can be found at World Time Zones
European clocks should re-adopt Summertime at 0100 UTC on 27 March 2022.
‘Daylight Saving Time’ in Canada and USA is not synchronised with Europe; it will finish on 7 November 2021 and restart on 13 March 2022.
In meteorological terms, Winter in the northern hemisphere will start on 1 December for 3 months. But, astronomically, Winter starts at the Winter Solstice, when the Sun reaches its most southerly latitude of 23º 26.1’ at 1558 UTC on 21 December.
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