Greenwich Mean Time (GMT)

Clock time and time zone standard

GMT uses the natural rotation of Earth as a universal timepiece

Time at the Shepherd Gate Clock, at the Royal Observatory in Greenwich, UK

GMT is based on regular observation of the rotational velocity of the Prime Meridian

Royal naval college, Greenwich
Royal Naval College (public domain)

UTC is rectified to remain within two seconds of GMT (leap second), with sub-second interval accuracy as prime objective

atomic clock
1955 caesium atomic clock

Greenwich Mean Time is two things

  • Greewich Mean Time = mean or average solar time at the Prime Meridian (0 degrees longitude)

    At noon, when the sun is straight above Greenwich meridian, the clock at the Royal Observatory shows 12.00.

    The word 'mean' is used to point out that GMT is an average of all the values recorded throughout a year.

    This is due to the specifics of the Earth's movement round its axis and round the Sun.

    Find out more about the history of GMT in this article

  • GMT is also the name of a time zone, used in UK and several other countries.

    Clocks in the UK show GMT during winter months when Daylight Saving (DST) is not in use.

    When DST is in use, between March and October, the clocks are on British Summer Time (BST), or GMT+1.

    The United Kingdom is in the Western European Time Zone.

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