Time is not singular, and in watchmaking this statement has precise mechanical consequences. There are at least three distinct concepts that coexist on the most elaborate dials: apparent solar time, mean solar time, and Coordinated Universal Time. Confusing them is the first mistake I see people make when approaching astronomical complications.
Apparent solar time follows the sun's actual position, which is not constant: Earth's elliptical orbit and axial tilt cause solar noon to fall earlier or later than a precise clock would indicate. The difference between the two—up to about sixteen minutes in certain periods of the year—is the so-called equation of time, a complication that displays this discrepancy in graphic form or via a dedicated hand. It's not a watch error: it's astronomical reality made readable.
Mean solar time, what our watches measure, was adopted because a mechanical movement cannot follow the irregularities of apparent solar time. From this came the system of time zones, codified in 1884 in Washington with Greenwich as the prime meridian. Zulu Time, in military and aeronautical terminology, is precisely this: the hour at the prime meridian, designated Z according to the NATO phonetic alphabet.
Timezone complications divide into well-defined families. GMT adds an independent twenty-four-hour central hand: the main hand stays on local time, while the GMT hand indicates the second timezone on a twenty-four-hour scale, typically on the bezel. Dual time instead uses a sub-dial or separate adjustable hand, without touching the main hours. The practical difference lies in adjustment: on classic GMT you move the additional hand in hourly steps; on dual time you shift the secondary indication without interfering with the main mechanism. Louis Cottier's world time takes this logic to its extreme: a rotating ring with twenty-four city names allows you to read all major time zones simultaneously.
Patek Philippe's travel time, with dedicated lateral pushers, and Breguet's patented system that updates hands, date, and day-night indicator with a single pusher, are different answers to the same problem: changing time zones without stopping the movement and without risking damage to the date mechanism during critical hours. The day-night indicator, often undervalued, is instead essential in dual time: knowing whether it's midnight or noon in the second timezone is more urgent information than the exact hour.
A side note I always find fascinating: the Martian sol, approximately forty-four minutes longer than Earth's day, has already produced dedicated watches. The Omega Speedmaster X-33 Marstimer handles the Martian time zone to coordinate surface missions with Earth control. It's watchmaking applied to space exploration, not science fiction.