Chronologically, the world time mechanism was developed and introduced prior to the now more common GMT complication. Created by the legendary Geneva watchmaker Louis Cottier, the world time watch was borne out of a need to redefine the mechanical watch during the Great Depression. Interestingly, Cottier had a close working relationship with both Rolex and Patek Philippe but it was Patek Philippe who applied for a patent for Cottier's world time system in 1959.

Reportedly inspired by the novelty of jet-powered flight and international telecommunications, Cottier pioneered the two-crown watch with 24-city ring on the dial that is still in use today. Briefly, the system is laid out this way: hour and minute hands in the centre for local time; a concentric disc around the central local time display that tracks the hours at the home city; and a an outer ring with the names of 24 cities in 24 different time zones.

Effectively, this means that one can get an idea of the time in 24 time zones all at once. It is worth noting that in cases of daylight savings time, the time shown will be incorrect. Nevertheless, this design proved so effective that most world time watches today are built around Cottier’s model. The Porsche Design watch pictured above is a twist, rather than an exception, with highlighted pointer synching with a location to provide the time in the aforementioned 24 time zones.

The fact that world time watches are quite rare also means that a lot of thought goes into these watches and they are often novelties in and of themselves.

In most cases, a world time watch works as follows: the crown at 3 o'clock controls the central hour and minute hands. A second crown or pusher handles operation of the city ring and the 24-hour ring. It may even control the central hour hand. All time functions are regulated by a single movement.

This complication is one of two that are the most popular exports from Switzerland (the other being the chronograph), according to the Federation of the Swiss Watch Industry. This contrasts strongly with the world time watch, which is rarity. Of course, the contrasts don't end there. Aesthetically, the world time watch and a watch with a second time zone, which we shall call the GMT watch, are totally at odds.

GMT watches are very clean and legible, unless of course there is another complication thrown into the mix. This clean and professional look – very nearly like an industrial tool – is part of the appeal of the GMT watch. Typically, the GMT watch has a central hour and minutes display for the local time, while home time or the second time zone is indicated via a second hour hand, which also operates out of the central hour axis but only makes a circuit of the dial once every 24 hours.

A common alternative to the GMT hand display is illustrated by the Piaget watch below. This watch features its second time zone on a subdial, complete with a day/night indicator.

All things being equal, there will be only one crown on a GMT watch. A rotating bezel is useful but not de rigueur. The addition of more than two time zones sometimes means that additional crowns will come into play.

History tells us that the GMT watch came into play at the dawn of the commercial jet age, classics here being from Rolex of course. The original GMT Master watch in 1954 was in fact the first GMT watch ever produced. Though this and other classic GMT watches use just one movement, some watches with more than just a second time zone actually feature different movements.