+05:30), the New York Stock Exchange opens at time in California = 09:30 − (−05:00) + (−08:00) = 06:30; time in India = 09:30 − (−05:00) + (+05:30) = 20:00. These calculations become more complicated near the time switch to or from daylight saving time, as the UTC offset for the area becomes a function of UTC time. The time differences may also result in different dates. For example, when it is 22:00 on Monday in Egypt (UTC+02:00), it is 01:00 on Tuesday in Pakistan (UTC+05:00). The table "Time of day by zone" gives an overview on the time relations between different zones. Nautical time zones Since the 1920s, a nautical standard time system has been in operation for ships on the high seas. As an ideal form of the terrestrial time zone system, nautical time zones consist of gores of 15° offset from GMT by a whole number of hours. A nautical date line follows the 180th meridian, bisecting one 15° gore into two 7.5° gores that differ from GMT by ±12 hours. However, in practice each ship may choose what time to observe at each location. Ships may decide to adjust their clocks at a convenient time, usually at night, not exactly when they cross a certain longitude. Some ships simply remain on the time of the departing port during the whole trip. Skewing of time zones Ideal time zones, such as nautical time zones, are based on the mean solar time of a particular meridian located in the middle of that zone with boundaries located 7.5 degrees east and west of the meridian. In practice, however, many time zone boundaries are drawn much farther to the west, and some countries are located entirely outside their ideal time zones. For example, even though the Prime Meridian (0°) passes through Spain and France, they use the mean solar time of 15 degrees east (Central European Time) rather than 0 degrees (Greenwich Mean Time). France previously used GMT, but was switched to CET (Central European Time) during the German occupation of the country during World War II and did not switch back after the war. Similarly, prior to World War II, the Netherlands observed "Amsterdam Time", which was twenty minutes ahead of Greenwich Mean Time. They were obliged to follow German time during the war, and kept it thereafter. In the mid-1970s the Netherlands, as other European states, began observing