How to Convert Time
In our previous discussion of ZD, we said that each standard meridian
(those meridians exactly divisible by 15) is 1 hour apart and that each of
these standard meridians is identified by a number and letter sign. To
convert ZT to GMT, or GMT to ZT, the first thing you must determine is
the correct ZD. ZT differs from GMT by the ZD.
Rule: When you convert ZT to GMT, you must apply the ZD to your ZT
using the proper sign, minus (-) if you are in east longitude, plus (+) if in
Example 1: Assume that you are in longitude 105°E, ZT is 16h 23m 14s,
and you want to find GMT.
Find your ZD. 105 ÷ 15 = 7.
Determine the sign of the correction. You are in east
longitude, so the sign is negative.
Apply the correction. Your ZD is -7. The minus sign
means that you subtract ZD from ZT to obtain GMT.
ZT 16h 23m 14s
G M T 0 9 h 2 3 m 1 4 s
Example 2: Assume you are in longitude 75°W, ZT is 07h 13m 57s, and
you want to find GMT.
Find your ZD. 75 ÷ 15 = 5. Therefore, you are in
Determine the sign of the correction. You are in west
longitude, so the sign is positive.
Apply the correction. Your ZD is +5, so add the
correction to ZT to obtain GMT.
GMT 12h 13m 57s