Louis Cottier patented the solution in 1931, and the underlying logic has remained unchanged ever since: a rotating ring with reference cities, paired with a 24-hour disc or scale, allows you to read the time simultaneously across all time zones without performing a single mental calculation. This is the substantial difference from GMT, which displays a single additional time zone via an extra hand — useful for those who frequently fly between two fixed points, but insufficient for those coordinating calls across three different continents.
The canonical Cottier system uses two concentric rings: the outer one bears the names of cities (one per time zone, twenty-four in total), while the inner one divides the day into light and dark arcs to distinguish day from night. Once your city is aligned with the current local time, you can instantly read the time in every other zone. Patek Philippe industrialized this principle in the caliber 240 HU — where HU stands for Heures Universelles — with a patented single-push system that advances the local hour, city ring, and 24-hour disc together without stopping the movement.
Over time, manufacturers have reinterpreted the mechanics in ways sometimes quite distant from the original. Bovet in the Récital 30 replaces the discs with twenty-six black PVD rollers: twenty-four for the cities, one for the UTC period, one for world time. Two side pushers respectively govern the rotation of the rollers by 90 degrees — to adapt to the four UTC seasonal periods, AST, EAS, EWT — and the advancement of the time zone by zone. Nomos in the Club Sport Neomatik uses a pusher at 2 o'clock to scroll IATA codes on the city ring, with a 24-hour sub-dial at 3 o'clock and a fixed ± scale to calculate the remaining time zones. These are different architectures answering the same question.
One detail I often see overlooked: some city rings display a solar symbol next to certain names to signal zones that observe seasonal daylight saving time. It is not mere decoration — it is operational information. Anyone working with American and European markets knows that during transition days the gap between time zones shifts by an hour, and a ring without that indication still forces you into mental conversion.
Modules for this complication are often supplied by external specialists: Dubois-Depraz is the most recurring name in this segment, with modules that integrate onto standard bases like the Sellita SW200. This does not reduce the complexity of dial design, which in a good world time watch remains arguably the most difficult graphic challenge in the entire industry.