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. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.
GMT in legislation
- 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
- Canary Islands (Spain)
- Most of Portugal, including Lisbon, Porto, Braga, Aveiro, and Coimbra. (Only the easternmost part, including cities such as Bragança and Guarda, lies east of 7°30'W.) Since the Treaty of Windsor in 1386 (the world's oldest diplomatic alliance), Portugal has maintained close ties to the UK, which possibly explains its choice of UTC. Madeira, even further to the west, also employs UTC. A more likely explanation is that during the mid-1970s, when Portugal was on Central European Time all year round, it did not begin to get light in Lisbon in winter until 08:30.
- The western part of Ireland, including the cities of Cork, Limerick, and Galway
- Westernmost tip of Northern Ireland, including the county town of County Fermanagh, Enniskillen
- Extreme westerly portion of the