Date line. When steaming from West to East time is lost for each time zone. Conversely, when proceeding from East to West time is gained.
The international date line running through the Pacific Ocean (about the 180 degree Meridian) indicates the place for change of date. When crossing the date line proceeding westward, the date must be advanced by omitting one day.
Conversely, when proceeding in an opposite direction there will be a loss of time and the date must be put back one day by repeating the same day and date.
It should be noted that in order to keep islands belonging to one group, for example, Aleutian Islands and the Tongan Islands on the same date, the 180 Meridian is not exactly followed by the date line.
