The International Date Line (IDL), is an imaginary line that runs roughly along the 180° line of longitude, in the middle of the Pacific Ocean. International convention accepts the line as the location where one day is divided from the next, with the area in the Eastern Hemisphere one day ahead of the Western Hemisphere. This line is necessary to address certain oddities that occur during travel; people going all the way around the world perceive themselves either gaining or losing a day, depending on which direction they traveled in.
How Time Zones Work
In theory, the world could be divided up into 24 time zones, one for each hour of the day. Time zones are actually set by each country independently, however, and local time does not necessarily match these ideal standard time divisions. Ideally, the time zones are designed so that local noon falls when the sun is roughly overhead. Traveling west, a person would lose an hour for every 15° of longitude; while traveling east, towards the sun, he or she gains an hour.
Time zones are described in terms of their relationship to the prime meridian, which falls at 0° of longitude in Greenwich, England. If someone was 45° away from Greenwich, he or she would experience a three-hour time difference; 45° to the west would mean that the person was in Coordinated Universal Time (UTC) -3, while someone 45° to the east would be in UTC +3. The International Date Line falls directly opposite the Prime Meridian in the time zones UTC +12 and UTC -12. Coordinated Universal Time was formerly known as Greenwich Mean Time (GMT), and is still sometimes written with this abbreviation.
How the IDL Location Was Chosen
The location of the International Date Line was chosen because it is in a relatively uninhabited area of the Pacific. The presence of inhabited areas — including Alaska, Russia, and a number of islands — mean that the line deviates in some areas. If the line was straight, the eastern most part of Russia would be a day behind the rest of the country, while a number of islands that are part of Alaska would be a day ahead of the state. This would likely result in confusion and administrative chaos.
Moving the Line
The line has been moved several times put a country on one or the other side. The islands of the Philippines, for example, were on the eastern side of the Date Line — with the Western Hemisphere — until the mid-19th century, despite sitting at about 120° E longitude. At the end of 2011, the islands of Samoa and Tokelau, which are very close to the line, shifted from the east to the west side of the line, rejoining the Eastern Hemisphere. Such changes are often made for economic reasons; Samoa and Tokelau shifted because of growing trade connections with Australia and New Zealand.
Crossing the Line
Traveling across the International Date Line can be confusing, especially for people taking a short trip, such as from Fiji to Hawaii. According to the clock, a traveler would end up arriving in Hawaii before he or she had left Fiji, because Fiji is a day ahead of Hawaii, and two hours behind — when it is noon in Hawaii, it is 10:00 AM in Fiji on the next day. Confusions of days and schedules do sometimes lead to mishaps, but most airlines and travel agencies keep the line in mind when informing travelers about schedules, expected arrival times, and itineraries.