The IDL is a vertical line that follows, with a few exceptions, the 180-degree longitudinal meridian from the north pole to the south. This means it cuts through the Pacific Ocean and runs past Pacific islands groups such as Hawaii, which is just to the east of the line; Fiji; and Tonga. The IDL also separates the eastern and western hemispheres.
The IDL is not completely straight: at certain points it deviates slightly from the 180th meridian, to avoid cleaving countries in half. For example, it deviates eastward through the Bering Strait to avoid dividing Siberia and then shifts westward to include the Aleutian Islands with Alaska. It makes another eastward deviation to include Fiji, Tonga, and Samoa in the same time zone as New Zealand.
The arbitrary division of the IDL means that anything to its east is one day behind, while anything to its west is one day ahead. It is set so that local noon time corresponds more or less to the time when the sun crosses the local meridian of longitude.
While the issue of an International Date Line first surfaced in the 12th century, it was at an 1884 conference that representatives of the world's nations gathered to formally establish the IDL. It was slightly adjusted in 1994 to accommodate some very tiny islands in the South Pacific.
