There are at least six different ways to display a second time reference on a mechanical watch, and confusing them is an expensive mistake when choosing or describing a piece. The starting distinction is between time zone and time measurement system: GMT, Dual Time, Travel Time, and World Timer complications concern time zones; equation of time and sidereal time, on the other hand, concern the very nature of the time being measured.
GMT is the most widespread solution: a supplementary hand completes a rotation in 24 hours and is read on a bezel or graduated dial, allowing you to keep track of a second time zone while living in the first. The distinction that matters in practice is between office GMT and traveler GMT: in the former, the GMT hand is adjusted in hourly increments while the local hour hand remains fixed; in the latter, the local hour hand is moved without touching the GMT hand or stopping the movement. At the counter, buyers unfamiliar with this difference often end up with the wrong instrument for their travel habits.
Dual Time is the simplest version: a second hour hand, often red, rotates overlapping the first. It works. It's understandable at a glance. It can hide behind the main hand when not needed. Travel Time is essentially the same idea, with brand names varying by manufacturer.
World Timer extends the logic to all time zones simultaneously. The classic version, codified by Louis Cottier in the 1930s, uses a ring with twenty-four cities and an inner ring divided into day and night segments. Some modern implementations add a symbol indicating cities observing daylight saving time, a useful solution but one that doesn't solve the problem mechanically: it's merely a visual reminder. The Bovet Récital 28 Prowess 1 is the only case I know of in which daylight saving time is managed with a roller mechanism, four different positions for twenty-four time zones.
Equation of time shows the difference between true solar time and mean solar time: up to sixteen minutes fast or slow, due to Earth's elliptical orbit and axial tilt. Mean solar time is what our clocks show; true solar time is what a sundial shows. The variant called Équation Marchante, found for example in the Breguet 5887, displays both in real time with separate hands, which requires a cam shaped according to the annual progression of the equation.
Sidereal time is yet another system: a sidereal day lasts 23 hours, 56 minutes, and 4 seconds, Earth's actual rotation period relative to fixed stars, not relative to the Sun. It appears almost exclusively in astronomical watches intended for night sky observation. It's not a travel complication: it's a scientific instrument in watch form.