Time and Date for Ships at Sea
As your ship travels east or west at sea and passes between one time zone
and the next, it is convenient for you (and everyone else on board) to
adjust the ships clocks to the time zone where you are actually located.
As you pass from one time zone to the next, ZT changes by 1 hour. But
do you advance the clocks 1 hour, or do you set them back 1 hour? The
If you are traveling towards the west, the new ZT will be 1 hour earlier;
therefore, you must set the ships clocks back 1 hour.
If you are traveling towards the east, ZT will be 1 hour later; therefore,
you must set the ships clocks ahead 1 hour.
The ships navigator or quartermaster should notify the commanding
officer when these changes become necessary. Do NOT, in any case, ever
advance or retard the ships chronometer.
So far weve been talking about advancing or retarding clocks to account
for time zone changes as we travel over the oceans. Suppose your ship is
in the Pacific Ocean traveling west. As you continue to travel west, you
are setting your clocks back 1 hour each time you enter a new time zone.
Eventually, you will lose 24 hours in a circumnavigation of the Earth.
Because of this, a method for adjusting for the day lost (or gained when
you were traveling east) is necessary and is accomplished by the
International Date Line, which follows the 180th meridian. The rule for
changing date when crossing the International Date Line is:
When traveling east and crossing the International Date Line, you
compensate by retarding the date 1 day.
When traveling west and crossing the International Date Line, you
compensate by advancing the date 1 day.
Note: The date change is in the opposite direction to the hour changes
you made as you passed into each new time zone. This date change is
made by every vessel that crosses the International Date Line, regardless
of the length of the voyage.
The International Date Line is used as a convenience just like time zones.
Changing the date should take place at a convenient time that is least
disruptive to the operation of your ship.