The case, winding-crown, push-pieces and crystal are so constructed as to be able to resist penetration by dust or water. A pressure of 3 atmospheres is equivalent to a pressure of 3 kg/cm² and corresponds to the pressure experienced by a watch submerged to a depth of 30 meters below the water’s surface. A water-resistant wristwatch (étanche) should be rechecked and serviced annually to preserve its impermeability to water.
As one travels eastwards or westwards from zero degrees longitude (the Greenwich or “prime” meridian), one’s local time deviates from GMT by one full hour for every 15º traversed. This world time system was first introduced by Canada and the USA in 1883. The dials of watches with World time indication display the time in two or more time zones. To accomplish this, it is necessary either to put several watch-movements inside a single case or else to add an additional mechanism. (A so-called “heure universelle” can simultaneously show as many as 24 different times.) World time wristwatches were first made in the 1930s. They are particularly popular with long-haul pilots, as well as with businesspeople who frequently make long-distance telephone calls from one time zone to another.