The website GreenwichMeanTime.com displays the exact Greenwich Mean Time. The local time in each time zone is defined by reference to the Greenwich Mean Time, which does not vary throughout the year.
The Greenwich Mean Time has been used since 1884, when 27 countries decided to adopt a time standard at the Meridian Conference. It is called Greenwich Mean Time because the Prime Meridian, from which east and west are defined, goes through Greenwich, now part of London. Greenwich Mean Time refers to solar mean time at the Royal Observatory in Greenwich. The time standard is called Universal Time, or UT.