How a GMT watch works

A GMT watch shows two time zones at once: normal local time on the standard 12-hour hands, plus an extra 24-hour hand that points to a second time zone on a 24-hour scale (usually the bezel). The name comes from Greenwich Mean Time — the original reference zone. The complication was made famous by the Rolex GMT-Master, developed with Pan Am for transatlantic pilots in the 1950s.

Read local time from the regular hour and minute hands as usual. For the second time zone, read the arrow-tipped 24-hour hand against the 24-hour scale: because it makes only one revolution per day, there is no AM/PM ambiguity — 06 is morning, 18 is evening. The day/night halves of the bezel make it readable at a glance.

On a "true" (flyer) GMT, the local 12-hour hand jumps in one-hour steps independently — perfect when you land somewhere new: click the hour hand to local time without stopping the watch. On a "caller" (office) GMT, it is the 24-hour hand that adjusts independently — better for keeping home time while checking a colleague's zone before calling. Both display the same information; the difference is which hand sets independently.

Yes, if it has a rotating 24-hour bezel. Keep the 24-hour hand on home time read against the inner scale (or the bezel at zero), then rotate the bezel by the offset of a third zone — the same hand now reads that third zone against the rotated bezel.

Because a 12-hour hand cannot tell 7 AM from 7 PM. Gearing the extra hand to one revolution per 24 hours removes the ambiguity entirely — essential when the second zone is somewhere your body clock has no feel for.

A GMT shows one extra time zone with a dedicated hand. A world timer shows all 24 zones simultaneously, using a rotating 24-hour ring aligned against a city ring. World timers are denser and usually pricier; GMTs are simpler to read and set.