Jagged red line down the right is the International Date Line

The International Date Line is an imaginary line on the surface of the Earth. Its purpose is to offset the hours that are added as one travels East through each successive time zone. It is for the most part at ±180° Longitude, on the side of the Earth that lies opposite the Prime Meridian, but has an odd shape to pass around Russia and islands in the Pacific.

The first phenomenon to occur in association with the date-line problem was part of Magellan's circumnavigation of the globe. The crew returned to a Spanish stopover sure of the day of the week, as attested by various carefully maintained sailing logs. Nevertheless, those on land insisted the day was different. Although readily understandable, this phenomenon caused great excitement at the time, to the extent that a special delegation was sent to the Pope to explain this oddity to him.

For the most part, the International Date Line follows the longitude line of 180 degrees. The two largest deviations from this meridian both occur to keep nations from crossing the date line internally. In the North Pacific, the date line swings to the East through the Bering Strait and then West past the Aleutian Islands in order to keep both Alaska (part of the United States) and Russia both completely on opposite sides of the line. In the central Pacific, the date line was moved in 1995 to extend around, rather than through, the territory of Kiribati. Prior to this time zone change, Kiribati straddled the date line; as a consequence, government offices on opposite sides of the line could only communicate on the four days of the week when both sides experienced weekdays simultaneously. A secondary consequence of this time zone revision was the new status of Caroline Atoll as the Easternmost inhabited land to enter the year 2000, a feature which the Kiribati government capitalized upon as a tourist draw.

However, even into the 2000s, many mapmakers do not account for this Kiribati line shift, preferring to continue to represent the International Date as a straight line in the Kiribati area.

The International Date Line can cause confusion among airline travelers. The most troublesome situation usually occurs at short distances from west to east. For example, to travel from Tonga to Samoa by air takes approximately two hours. Thus, if a person leaves at noon on Tuesday, they will arrive at 2 PM on Monday. Meanwhile, someone in Samoa inquiring about the departing flight may be told there is no flight until the next day. There could also be problems with the traveler having to repeat Monday. Journal entries and photographs may end up out of sequence, and there could be errors in someone's medication schedule. In addition, those making ownward airline connections might chose the wrong date for the reservation.

The effect of ignoring the date line is also seen in Jules Verne's work of fiction Around the World in Eighty Days, in which the travellers return to London after a trip around the world, thinking that they have lost the bet that is the central premise of the story. Having circumnavigated in the direction opposite Magellan's, they believe the date there to be one day later than what it truly is.

Anyone travelling West and passing the line must add a day to what they would otherwise expect the date and time to be. Correspondingly, those going East must subtract a day. Magellan's crew and Verne's travellers neglected those adjustments, respectively.

External link

• A History of the International Date Line