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Greenwich Mean Time
time zone
World Time Zones Map.png

Ambiguity in the definition of GMTPtolemy, was to refer to noon as zero hours (see Julian day). This contrasted with the civil convention of referring to midnight as zero hours dating from the Roman Empire. The latter convention was adopted on and after 1 January 1925 for astronomical purposes, resulting in a discontinuity of 12 hours, or half a day. The instant that was designated "December 31.5 GMT" in 1924 almanacs became "January 1.0 GMT" in 1925 almanacs. The term Greenwich Mean Astronomical Time (GMAT) was introduced to unambiguously refer to the previous noon-based astronomical convention for GMT.[11] The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.

GMT in legislation

United Kingdom

  • The westernmost part of Iceland, including the northwest peninsula (the Westfjords) and its main town of Ísafjörður, which is west of 22°30'W, uses UTC. Bjargtangar, Iceland is the westernmost point in which UTC is applied.

Countries and areas west of 7°30'W ("physical" UTC−1) that use UTC