What Is The International Date Line?

The North Pole, where the IDL originates.

The International Date Line is an imaginary. It runs from the North Pole to the South Pole and is used as a demarcation of calendar days. The International Date Line primarily runs along the 180-degree longitude which is in the middle of the Pacific Ocean. However, the International Date Line deviates from the longitude to pass through several groups island in the ocean. The International Date Line has no international legal status with countries free to choose their own.


The International Date Line originates from the North Pole and runs down the 180-degree longitude. The line first deviates from the 180-degree longitude at the Chukchi Peninsula and the Wrangel Island and later passes between the Diomede Islands in the Bering Strait where it diverges west from the longitude. The International Date Line then divides the Aleutian Islands of the United States and the Commander Islands of Russia effectively making all of Russia to be west of the International Date Line and all of the United States (except a few islands) to be east of the International Date Line. The International Date Line then runs on the 180-degree longitude until it reaches Kiribati where it deviates east almost touching the 150-degree longitude. The line then returns on the 180-degree meridian until it reaches Samoa where it diverges and follows the 165-degree longitude on this particular region of Polynesia and then reverts to the 180-degree meridian until it reaches the South Pole in Antarctica. The numerous diversions of the International Date Line along the 180-degree meridian (which appear like a huge zigzag on the map) are primarily for political purposes with the normal standard of changing in dates in aviation and maritime travel being purely along the 180-degree longitude.

History of the International Date Line

Before the advent of clocks, time was measured according to the sun’s position with a day being the duration between two consecutive “noons.” The early mechanical clocks were set to use this particular description of time, but their effectiveness was limited to a specific geographic area due to the variations in the position of the sun in different regions. During the 19th century, the transport industry was experiencing a boom with trans-continental travel being made easier and faster. However, the long-distance travel posed an issue with time-keeping with travelers having to set their clocks severally during their journey which was an annoying inconvenience. The development of other long-distance modes of telecommunication such as the telegraph and the telephone further compounded the problem especially in their military and commercial application. The remedy for this problem was to establish a global standard of time with 24 time zones, a theory which was first coined by Sir Sandford Fleming in 1876. The theory was applied in 1900 and used to date. Standard Time Zones were divided to be 15 degrees wide with the Greenwich Meridian, at zero degrees, being the starting point. The time zones run east and west of the Greenwich Meridian until they reached the 180 degrees longitude which became the International Date Line.

Use of the International Date Line

Common knowledge states that the day and date change at midnight regardless of the position on earth. However, scientists thought it would be best to have a longitude to be the basis of the change in date. The 180-degree longitude was chosen as the International Date Line due to its position on the UTC (Universal Coordinated Time) where it is the only longitude which lies at either UTC 12+(plus) or UTC 12-(minus). Another reason why the longitude was chosen to be the International Date Line was its remote location as it runs through the middle of the Pacific Ocean where few habitable islands exist. If one crosses the International Date Line while traveling west, the day and date increases by one unit. On the other hand, if the International Date Line is crossed while traveling east, the day and date decrease by one unit. Therefore once one crosses the International Time Zone, he or she is in theory time traveling.

De facto and De jure

The location of the International Date Line can be defined using two different criteria; de facto and de jure. In maritime and aviation travel, the location of the International Date Line (nautical date line) is critical and therefore dictated by an international agreement made during the 1917 Anglo-French Conference on Time-keeping at Sea. The agreement makes the IDL a de jure determination and strictly follows the 180-degree longitude from pole to pole with few alterations in areas with territorial waters being close to land. The nautical date line is used by ships and long-range airplanes. On the other hand, in the de facto definition, the International Date Line is defined with its implications being primarily on land and adjacent territorial waters. This definition of the International Date Line bears no legal recognition with countries able to set time zones at their discretion. A good example of this application is China whose huge land area makes it cover several time zones but according to the government, covers one time zone over its entire area.

Application of the International Date Line

Countries are not obliged to use the International Date Line as a standard in setting dates. Some scientists view the International Date Line as a political tool used to define political affiliations especially to distinguish island territories of the United States from those belonging to Russia. An excellent example of the geopolitical application of the International Date Line is the Samoan Islands which were initially on the west of the International Date Line until the islands were divided into Samoa and American Samoa with American Samoa adopting the American date and therefore went east of the International Date Line. The distance between these two islands is about 100 miles, but in theory they are located on different dates and days. However, the ideal purpose of the International Date Line is to prevent the overlapping of dates. The International Date Line is recognized across major religions in the world including Christianity, Islam, and Judaism.


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