from
The Collaborative International Dictionary of English v.0.48
Date line \Date line\
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. Also
called {International Date Line}.
[Webster 1913 Suppl. +PJC]
Note: Speaking generally, the date line coincides with the
meridian 180[deg] from Greenwich. It deflects between
north latitudes 80[deg] and 45[deg], 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[deg] and 56[deg], 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 Suppl.]