The hypothetical line on the surface of the earth fixed by international or general agreement as a boundary on one side of which the same day shall have a different name and date in the calendar from its name and date on the other side.
⇒ Speaking generally, the date line coincides with the meridian 180° from Greenwich. It deflects between north latitudes 80° and 45°, so that all Asia lies to the west, all North America, including the Aleutian Islands, to the east of the line; and between south latitudes 12° and 56°, so that Chatham Island and the Tonga group lie to the west of it. A vessel crossing this line to the westward sets the date forward by one day, as from Sunday to Monday. A vessel crossing the line to the eastward sets the date back by one day, as from Monday to Sunday. Hawaii has the same day name as San Francisco; Manila, the same day name as Australia, and this is one day later than the day of Hawaii. Thus when it is Monday May 1st at San Francisco it is Tuesday may 2d at Manila.
© Webster 1913