Time is measured in terms of the rotation of the earth. Since the earth rotates 360º in a 24-
hour period, we divide 360 by 24 to yield 15ºof rotation in one hour. This divides the earth
into 24 time zones; each 15º of longitude in width, making the time between each zone
differ by one hour. Each time zone is centered on a meridian that is a multiple of 15º. The
time within each zone is called Local Mean Time (LMT).
Figure 4.2-11 Time Zones
Each time zone has been given both alphabetic and numeric designators. The alphabetic
designator for the time zone centered on the zero-degree meridian (the prime meridian) is
"Z" (Zulu). The time within the Zulu time zone is called Greenwich Mean Time (GMT).
Greenwich Mean Time is used as the reference for each of the remaining zones. The zone
description (ZD), numeric designator for any zone, indicates the difference in hours from
local time to GMT.
In air travel, where great distances can be covered in a short time, it is inconvenient to
keep track of time zones being crossed. To avoid confusion, Greenwich Mean Time is the
standard used for aviation since GMT is the same all over the world at any particular
instant in time. For example, weather briefs and flight plans are filed using GMT. Therefore,
you must be able to convert any local time to GMT and GMT to local time.