At sea, as well as ashore, watches and clocks are normally set to some form of zone time (ZT). At sea the nearest meridian exactly divisible by 15° is usually used as the time meridian or zone meridian. Thus, within a time zone extending 7.5' on each side of the time meridian the time is the same, and time in consecutive zones differs by exactly one hour. The time is changed as convenient, usually at a whole hour, when crossing the boundary between zones. Each time zone is identified by the number of times the longitude of its zone meridian is divisible by 15°, positive in west longitude and negative in east longitude. This number and its sign, called the zone description (ZD), is the number of whole hours that are added to or subtracted from the zone time to obtain Greenwich mean time (GMT). The mean sun is the celestial reference point for zone time. See figure below.

 

Converting ZT to GMT, a positive ZT is added and a negative one subtracted; converting GMT to ZT, a positive ZD is subtracted, and a negative one added.

Example: The GMT is .

Required:

(1) ZT at long.	156°24.4' W.
(2) ZT at long.	039°04.8' E.

Solutions:

(1)

GMT
ZD	(rev.)
ZT

 

(2)

GMT
ZD	– (rev.)
ZT

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