Back to zulu time
At 0100Z this morning, 31 October, UK civil time moved back from British Summer Time (BST) to Coordinated Universal Time (UTC).
A European Parliament and Council Directive states that summer (or daylight-saving) time will be kept throughout the EU between the last Sunday in March and the last Sunday in October. The changes will take place at 0100 UTC.
Hence civil time will change back to summer time at 0100 UTC on 27 March 2011.
Whilst Europe changes civil time in harmony, much of the rest of the world does not - most of the US and Canada, for instance, will change a week later on 7 November.
Coordinated Universal Time (UTC - the acronym is a fudge between the English and French versions and is correct in neither) is based on the average of a series of atomic oscillators, with a leap-second added around every 500 days to keep it synchronised with the earth's rotation. For practical navigational purposes, Coordinated Universal Time can be considered identical to Greenwich Mean Time.
There are 24 lettered time-zones around the world, each centred on a multiple of 15º longitude. GMT is annotated Z (zulu), which is used for international navigation and timing to prevent ambiguity and confusion between local times. Central Europe, based on 15ºE, uses A (alpha) - which also becomes British Summer Time.
Details from the National Maritime Museum below.