Managing time zones in watchmaking is not a single function: it's a family of distinct mechanical solutions, with different geometries and opposing philosophies of use. From the counter, the first thing I tell anyone looking for a GMT is: do you want to change the time zone yourself, or do you just want to consult a second one? From there, everything becomes clear.
The fundamental distinction is between GMT traveller and GMT office. In a GMT traveller, the hour hand moves in one-hour increments without stopping the seconds or minutes: someone crossing time zones during travel adjusts the local time without touching the GMT hand, which remains anchored to the reference time. In GMT office, it's the opposite: the 24-hour GMT hand is the independent one, while local time doesn't move. Useful for someone staying put but keeping an eye on a second location. Confusion between these two types is the number one source of post-purchase disappointment.
The GMT of the original Rolex GMT-Master I was a third type: the GMT hand was mechanically linked to the hour hand, so adjusting local time also shifted the GMT reference. It wasn't a true independent dual-time. Rolex solved this in subsequent calibres. Today this architecture is called coupled GMT, to distinguish it from caller-style, where the two hands are kinematically separate.
The worldtimer is a further level up: it shows all major time zones simultaneously, usually with a disc or ring displaying city names. Real complexity emerges when managing half-hours, quarter-hours, and four variants of daylight saving time: on this, most commercial worldtimers make compromises. Only a few manufactures — Bovet is a documented case — have addressed the entire range mechanically.
The fixed 24-hour bezel, like on the Tudor Black Bay Pro, is the most robust and least expensive solution: no additional gearing, no risk of accidental adjustment. You read the second time zone by pointing the GMT hand at the bezel scale. It works, but requires that the GMT hand exists in the movement.
The flyer disc, used for example in the Miyota 9075, replaces the hand with a coloured sector on a rotating disc under the dial: readable at a glance, less precise for instantaneous reading, but aesthetically clean.
On solar GPS applied to time zones: the Seiko 5X83 calibre synchronises via satellite and automatically sets the correct time zone. It's not a mechanical complication; it's an electronic solution that solves the same problem in a radically different way. Precise, but different in nature.