· By Buffy
GMT and World Time
Second time zones without a second watch: the extra hour hand, the 24-hour bezel, and Louis Cottier's world-time mechanism.
Time zones are a political-geographic mess, so of course watches built mechanisms to hold several at once. Two families dominate: the GMT watch (one extra hour hand) and the world timer (all 24 zones at a glance).
The GMT watch
A fourth hand — the GMT hand — geared to the motion works makes one revolution per 24 hours. Read it against a 24-hour bezel or an inner 24-hour track: it points at the home zone’s hour while the normal hands show local time.
The archetype: the Rolex GMT-Master (1955), designed with Pan Am for transatlantic crews who needed home and destination time simultaneously. Its 24-hour bezel (two-color Bakelite originally) isn’t a toy — it distinguishes day from night in the home zone, which matters when you call the office.
Mechanically, the GMT hand is almost free: one extra wheel ratio off the hour wheel. Modern “true” or “flyer” GMTs (local-hour-jumping) add a mechanism to jump the local hour hand in whole hours when crossing zones — the traveler’s GMT — while “caller” GMTs move the GMT hand instead. Purks argue; both are gear trains.
World time
The world timer shows all 24 zones at once: a city ring, a 24-hour ring rotating once per day, and the local city at 12 o’clock. The design is due to Louis Cottier (1930s Geneva watchmaker), whose mechanism is delightfully simple — the 24-hour disc is driven off the hour wheel; the city ring is set by a pusher or bezel.
Reading it: find your city on the ring; the hour it aligns with on the rotating 24-hour disc is the current hour there. Daylight saving is the one thing it can’t know — world timers assume standard offsets, so the display is a political snapshot, not a live database. (Everything is a trade-off; see What Counts as a Complication.)
Complications that count zones
Both designs are, at heart, counting mechanisms: GMT counts a 24:1 ratio; world timers map 24 hours onto 24 labels. The perpetual calendar counts months; the same gear-train honesty applies. And all of them go wrong only where politics does: borders, half-hour zones (India, Newfoundland), and daylight saving.