The time-conversion table has 24 horizontal rows depicting the 24 hours of the day, and 25 vertical columns showing the 25 time zone designators. See table 2-1. Notice that zones MIKE and YANKEE are identical, with the exception of the day.
To use the time-conversion table, find the zones in question along the horizontal row at the bottom of the table and go up the vertical column of the known time zone. Then find the corresponding vertical position of the unknown zone. You now have the time of the unknown zone in relation to the known zone. It's as simple as that.
COMMERCIAL TIMECONVERSION AIDS
The commercially produced timeconversion aids, primarily designed to aid the tourist, are inadequate for military and communications use. They generally disregard zone designators and the computation processes. Figure 2-1 shows a typical tourist-oriented, time-conversion aid and is included in this manual only as an example of these aids.
We have discussed time-conversion working aids only to advise you that there are shortcuts. There are no shortcuts to professionalism, however, and each of the time-conversion aids has its shortcomings. Did you notice that the time-conversion table is of no help in establishing positional locations of targets? Additionally, if you are on a directsupport platform, or at an isolated duty station where the time conversion table is not available, the success of your mission might well depend upon your ability to compute time.
If any one of these areas is unclear to you, go back to the discussion and master that procedure.
1. To determine ZULU time from local time, apply the sign ("+" or "-") and add or subtract the numerical designator to or from the local time's hours.
2. To determine local time from ZULU time, reverse the sign ("+" or "-") and add or subtract the new numerical designator to or from the ZULU time's hours.
3. In problems involving geographical positions:
a. Latitude is irrelevant for figuring time; use only the longitude.
b. Use all five digits of the longitude (seven digits, if given).
c. Proceed in an easterly or westerly direction from the prime meridian, according to the "E" or "W" designation.
d. Make a rough, graphic chart to establish the zone in which a given longitude falls.
(1) Enter the longitudinal coordinates for the ZULU zone (00730'E or 00730-W).
Figure 2-1.-Typical commercial time-conversion aid.
(2) When traveling from the eastern or western border of zone ZULU, add 15 for each new zone; place this new longitudinal coordinate at the eastern or western meridian of the zone, as required.
e. Solve the time problem like any other problem after placing the target into the zone corresponding to its longitudinal coordinates.
4. In problems involving the use of a target's local time to establish its longitudinal parameters:
a. Convert your local time to ZULU.
b. Work from ZULU time to derive the local time of the target.
c. Place the target within its geographic zone once the local time is determined.
5. In problems involving the International Date Line:
a. Separate the MIKE and YANKEE zones.
b. Label both "+" and "-" designators (MIKE is "-"; YANKEE is "+").
c. It is always the same time in MIKE as it is in YANKEE, but never the same day.
d. The day must change each time the International Date Line is crossed.
e. Apply the sign of the departed hemisphere when crossing the line to determine whether to add or to subtract a day.
6. In problems involving daylight saving time (DST):
a. When time is given in DST, work the problem in normal fashion, then subtract 1 hour to arrive at the zone's normal time.
b. When the zone's normal time is given, work the problem in the usual fashion, then add 1 hour to determine DST.
Communications Instructions General, ACP 121(F), Annex A, Joint Chiefs of Staff, Washington, DC, 15 April 1983.