At this point, we will discuss one more area needed for time calculation. It is directional flow and the addition or subtraction of an hour when progressing from one time zone into another. Probably the best way to remember whether to add or to subtract the hour is to take the case of the four time zones spanned by the United States (ROMEO through UNIFORM).
Most of us have, at some time or other, watched a sporting event being played on the West Coast while we were physically located on the East Coast. In cases where the contest was held in the late afternoon or early evening in California, it was frequently dark in New York. Obviously, it was earlier in the day in California than it was in New York. Therefore, we can say with confidence that whenever traveling from a westerly direction toward a point eastward, we must add an hour each time we pass from one time zone into another. The opposite is also certainly true. When traveling from an easterly direction toward a point westward, we must subtract an hour for each new zone entered. This rule will hold true regardless of your location in the world: west to east-add, east to west subtract. Additionally, when the 0000 hour is reached, the day changes accordingly.
It is absolutely essential that you understand each of the points covered thus far in this manual before attempting to convert time. The following is a short review of these principles. Test yourself. If you do not fully understand any of them, go back and reread the related section.
1. The international Greenwich mean time (GMT) system was named for the town of Greenwich, England, as the town is located directly on the prime meridian, the point of reference for the entire system.
2. The surface of the earth is divided into 24 time zones, each spanning 15° of longitude.
3. The initial zone is zone 0 (ZULU) and spans the area 71/2' longitude east and 71/2' longitude west of the prime meridian (a total of 15°).
4. Each zone differs in time by 1 hour.
5. Each zone has a numerical, a literal, and a "+" or a "-" designator (exception: ZULU zone (0) does not have a "+" or "-" designator).
6. The zones are numbered 1 through 12, outwardly from zone 0, throughout both the eastern and western hemispheres.
7. The zones east of ZULU are lettered ALFA through MIKE, omitting JULIETT, and each has a "-" designator.
8. The zones west of ZULU are lettered NOVEMBER through YANKEE, and each has a "+" designator.
9. At the equator there are 60 nautical miles (NMs) in a degree and each time zone spans 900 NMs; a time zone spans 15° of longitude (exception: MIKE and YANKEE-each span 7 ˝” of longitude).
10. The U.S. Navy uses the international 24-hour time system, expressed in four digits; DTGs are formed by preceding the four-digit time with a two-digit number expressing the day.
11. The International Date Line separates the designators MIKE and YANKEE (-12 and +12). The date will always change when crossing this line, regardless of the direction of crossing. When you cross the line, apply the sign of the departed hemisphere.
12. MIKE and YANKEE are one time zone of 15° longitude, sharing the same numerical designator (12). MIKE is the eastern 71/2° of longitude of this zone; YANKEE is the western 71/2° of longitude.
13. The time will change by 1 hour whenever a new time zone is entered: east to west, subtract 1 hour; west to east, add 1 hour.
14. The day changes to the next or previous day once 0000 is reached, depending upon the direction of travel.
15. The time is always the same in MIKE as it is in YANKEE, but it is never the same day.
Communications Instructions General, ACP 121(F), Annex A, Joint Chiefs of Staff, Washington, DC, 15 April 1983.