GMT — Greenwich Mean Time

GMT is the historical time standard set at the Greenwich meridian, still widely used as a label.

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Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, London. For everyday purposes GMT and UTC show the same clock reading.

GMT is a time zone that some countries observe; UTC is a technical standard that no country observes directly. The United Kingdom uses GMT in winter and British Summer Time (BST, UTC+1) in summer — so "London time" and "GMT" are not the same thing all year.