TimeZones and international waters

[Paul Eggert]

To help clear this up I'll add something like the following to my next
proposed patch.  Comments welcome.

A ship within the territorial waters of any nation uses that nation's
time. In international waters, time zone boundaries are meridians
15° apart, except that UTC−12 and UTC+12 are each 7.5°
wide and are separated by the 180° meridian (not by the
International Date Line, which is for land and territorial waters
only). A captain can change ship's clocks any time after entering a
new time zone; midnight changes are common.