Dual time

A dual-time watch shows two zones at once. Rolex, describing its own GMT-Master II, states: "The GMT-Master II features an additional, triangle-tipped hand. This indicates a second time zone on the 24-hour graduated rotatable bezel." That is Rolex's own account of its own product, and note what the page does not do: it calls this the "GMT function" without anywhere defining the abbreviation or mentioning Greenwich Mean Time. No standard of ISO/TC 114, the ISO committee for horology, defines the dial term either: the eight it holds directly cover jewels, control-position symbols, wrist-chronometers, battery life, chemical compliance, hard-material external parts and watch batteries.

GMT as a timescale is a different thing from a second hour hand. Royal Museums Greenwich states that "Greenwich Mean Time is the yearly average (or 'mean') of the time each day when the Sun crosses the Prime Meridian at the Royal Observatory Greenwich", that "from 1884 until 1972, GMT was the international standard of civil time", and that "though it has now been replaced by Coordinated Universal Time (UTC), GMT is still the legal time in Britain in the winter". The timescale detail belongs to the entry for coordinated-universal-time.

The reference the zones are counted from is itself documented. Royal Museums Greenwich records that in 1884 delegates meeting in Washington "voted to make the Greenwich Meridian the world's Prime Meridian", and that "the cross-hairs in the eyepiece of the Airy Transit Circle precisely defined zero degrees longitude for the world". The museum's separate page "What is the Prime Meridian, and why is it in Greenwich?" adds that "Earth's current internationally agreed prime meridian is the IERS Reference Meridian", which "passes 102.5 m to the east of the historic Prime Meridian".