GMT / Dual-time
A second time zone shown on the same dial
What it is
A GMT complication adds a fourth hand that rotates once per 24 hours, pointing to a 24-hour graduated bezel or chapter ring. This allows the wearer to read both local time (from the standard three hands) and a second time zone (from the GMT hand) simultaneously. The 24-hour scale distinguishes AM from PM on the reference time zone; a detail the standard 12-hour dial cannot show. Dual-time watches achieve the same result via a sub-dial showing hours in the second zone.
History
Rolex developed the GMT-Master (reference 6542) in 1954 in collaboration with Pan American World Airways, whose pilots needed to track home time while operating across time zones. The original used a rotating bezel set to home time and a fixed GMT hand. In 1983 Rolex introduced the GMT-Master II with an independently adjustable local hour hand; allowing the wearer to set local time in one-hour jumps without stopping the movement. The Tudor Black Bay GMT (2018) brought functionally identical capability to a significantly lower price point. The two-colour Pepsi (red-and-blue) and Batman (black-and-blue) Cerachrom bezels required a breakthrough in co-sintering two ceramic colours in a single mould.
How it works
An additional wheel train driven off the cannon pinion rotates a 24-hour wheel, which in turn moves the GMT hand around the dial at one revolution per 24 hours. On a true GMT (Rolex-style), the local hour hand is driven by a quick-set mechanism that adjusts it in one-hour increments without stopping the movement or disturbing the running seconds hand. Setting local time when crossing time zones requires only a crown adjustment, not a full reset.
Parts required
24-hour wheel, additional cannon-pinion-driven intermediate wheel, GMT hand arbor, and on true GMT versions: an independent hour-hand setting mechanism with click-set detent
In the catalog
Related
- Worldtime: All 24 time zones displayed simultaneously
