GMT

GMT (Greenwich Mean Time) in watchmaking denotes a complication with an additional 24-hour hand and a corresponding scale — typically on a rotating bezel, sometimes on an inner ring or sub-dial. The wearer reads a second time zone alongside local time. With a rotating bezel a third time zone can be drawn in.

The name refers to the Greenwich reference time, which served as the global standard until 1972; since the introduction of coordinated universal time (UTC), "GMT" is technically obsolete but has stayed in watchmaking as a generic label.

Mechanical variants

Two constructions are fundamentally different in daily use:

• Caller GMT (office GMT). The GMT hand is independently adjustable, the hour hand is not. Suited to keeping home time on the wrist while the GMT hand reflects a call in another zone. Examples: Rolex GMT-Master 1675, earlier Tudor Black Bay GMT.
• Flyer GMT (true GMT). The hour hand jumps independently forward and backward in one-hour steps, the GMT hand continues to show home time. Suited to travellers: on landing, jump the hour hand to local time, GMT hand stays on home. Examples: Rolex GMT-Master II (from 1983), Patek Philippe Aquanaut Travel Time, Cartier Santos Dumont GMT.

The distinction is value-defining on the secondary market. Flyer-GMT movements are technically more involved and accordingly priced higher.

How the bezel plays in

A rotating 24-hour bezel allows three time zones to be read simultaneously. Example: local on the hour hand, home on the GMT hand against the fixed inner 24-hour scale, third zone via rotating the bezel against the GMT hand. The two-tone Pepsi and Batman configurations of the GMT-Master II distinguish day (light half) and night (dark half) on the bezel — a convention unchanged since 1955.

Which watches carry it

• Rolex GMT-Master II 126710BLRO Pepsi, 126710BLNR Batman, 126720VTNR Sprite, Explorer II 226570 with fixed 24-hour bezel.
• Patek Philippe Aquanaut Travel Time 5164A, Calatrava Pilot Travel Time 5524G.
• Omega Seamaster Aqua Terra Worldtimer, Seamaster GMT.
• Audemars Piguet Royal Oak Offshore Diver GMT, Code 11.59 GMT.
• Cartier Santos Dumont GMT, Santos GMT.
• Vacheron Constantin Overseas Dual Time, Overseas Worldtime.

The label "Worldtimer" — as in the Patek 5230 or the Vacheron Overseas — describes an extended GMT variant displaying all 24 time zones simultaneously via a rotating city disc.

Setting and service

Setting follows the construction. On a flyer GMT, home time goes on the GMT hand first, then the hour hand jumps to local time. On a caller GMT, local time is set first on the hour hand, then the GMT hand is set via the crown (second click position) to the second zone.

At our atelier in Munich we see two typical service cases: a GMT hand that no longer runs precisely synchronous with the hour hand after impact, and a crown that no longer takes torque in the second click position. Both are workshop tasks, not home repairs.