For reckoning time, the surface of the globe has been divided into 24 zones; each
is bounded by meridians of 15° of arc, and each is 1 hour in longitude. The initial
time zone lies between 7 1/2°E and 7 1/2°W of the Prime Meridian; it is called
ZONE ZERO. Each zone, in turn, is designated by the number that represents the
difference between local zone time and Coordinated Universal Time (UTC). See
figure AIII-1. Zones have been modified near land to accord with the boundaries of
the countries or the regions using corresponding time.
The zones lying east of zone zero are numbered 1 through 12 and are designated
minus. And for each of the minus zones, the zone number is subtracted from the
local time to obtain UTC. The zones west of zone zero are also numbered from 1
through 12 but are designated plus, since the zone number must be added to the local
time to get UTC. The twelfth zone is divided by the 180th meridian, the minus half
lying in east longitude and the plus half in west longitude. The zone number
preceded by a plus or a minus sign constitutes the zone description. In addition to the
time zone number, each zone is also designated by a letter A through M (J omitted)
for the minus zones, and N through Y for the plus zones. (See top of figure AIII-1.)
Date/time groups (DTGs) are frequently used to express times of specific
events, such as the time a message was written. A date/time group always contains 6
digits; The first 2 digits are the day of the month, and the last 4 digits are the time,
using the 24-hour clock. The appropriate time zone letter designator follows the 6-
Local times may be used in the text of a message or letter. When local time is
used, it must be accompanied by the zone letter-such as 08124. If local time is
referred to frequently in the text, the suffix may be omitted provided a covering
expression, such as "ALL TIMES QUEBEC", is used.
When it is necessary to indicate a date alone in a message, it is expressed by the
day of the month, the three-letter abbreviation of the month, and (if necessary) the
last two figures of the year: 3 FEB or 3 FEB 97. A night is expressed by the word
night and the two dates over which it extends: NIGHT 3/4 FEB 97.
TIME CONVERSION TABLE
The time conversion table (table AIII-1) is useful for converting the time in one
zone to the time in any other zone. Vertical columns indicate time zones. ZoneZ is
UTC. Time in each successive zone to the right of zoneZ is 1 hour later, and to the
left of zone Z is 1 hour earlier.
To calculate time in zone I when it is 1200 hours in zone U, for example, proceed
as follows: Find 1200 in column U and locate the corresponding time in the line in
column I (0500). Since both of the times (1200 in zone U and 0500 in zone I) are not
in the "same day" area of table AIII-1, the time 0500 in zone I is tomorrow to the
time 1200 in zone U. In other words, when it is 1200 in zone U, it is 0500 tomorrow
in zone I; or when it is 0500 in zone I, it is 1200 yesterday in zone U.