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|The International Date Line
is an imaginary line set at 180º longitude that "adds a day" when crossed going to the west -- that is, Sunday on the east side of the line becomes Monday as one crosses westward.
In 1884, members of the International Prime Meridian Conference agreed that the longitude of Greenwich, England, would become zero degrees longitude and established the 24 time zones relative to the "Prime Meridian." That same conference established 180º longitude as the International Date Line, since it is exactly opposite the Prime Meridian.
The International Date Line is not, however, an absolutely straight line. It zigzags, for political reasons, so that it never crosses the territory of any nation. For example, all of Siberia is in the Asian system, even though its easternmost peninsula should actually lie across the line. In the South Pacific, the line was adjusted so that all islands within any given political association were on one or the other side.
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This page was last updated on 01/22/2018.