EXPRESSION OF TIME
The U.S. military services, as well as most foreign countries, use the international 24-hour system for expressing time. This method uses a four-digit group, with the first two digits denoting the hour, and the second two digits indicating the minutes. Thus, 6:30 A.M. becomes 0630; noon becomes 1200; 6:30 P.M. becomes 1830. Midnight is expressed as 0000, never as 2400. One (1) minute past midnight is 0001. The time designation 1327Z shows that it is 27 minutes past 1:00 P.M., GMT.
To express the day of the month along with the time, we use a six-digit group. These six digits are nothing more than a four-digit time, preceded by two digits indicating the date. This six-digit group is a date-time group (DTG). The DTG 171327Z indicates the 17th day of the month at 1327Z.
The date element of the DTG always has two digits. This means the dates from the 1st through the 9th of the month must be preceded by a zero (0) to meet this requirement (for example, 011327Z, 021327Z, or 031327Z). Should a month other than the current one be intended, the standard abbreviation for the month desired follows the DTG (for example, 011327Z JAN, 121327Z FEB, or 211327Z MAR).
In each of the above examples, the times were expressed in ZULU time. This is to make you think in terms of ZULU, since ZULU zone time is the standard time for military communications . All messages, reports, and letters containing times, use ZULU time. This enables all mobile platforms and shore stations to know at what time the subject of the correspondence occurred. It becomes simply a matter of converting the ZULU time of the occurrence to the local time.
Obviously, there are occasions when time must be expressed as local. In these instances, the literal designator for the local zone is used in exactly the same manner as the ZULU designator was used. For example, in the UNIFORM time zone, 171327U would indicate the 17th day of the current month, 27 minutes past 1300 local time.
INTERNATIONAL DATE LINE
The International Date Line divides the eastern and western hemispheres. It is an imaginary line located exactly 180 east longitude and 180 west longitude of the prime meridian. At this point, we must understand the special circumstances surrounding zones MIKE and YANKEE.
Each time zone has a numerical, a literal, and a "+" or a "-" designator, and zones MIKE and YANKEE are not exceptions. There is, however, a very important difference between zones MIKE and YANKEE and all other time zones. To understand this difference, look at zones MIKE and YANKEE as a single time zone of 15 of longitude, half (7 ) in the eastern hemisphere, and half in the western hemisphere. Although the two halves of this zone share a common number (12) each half has its own literal and "+" or "-" designator. The eastern hemisphere's half is designated MIKE -12; the western hemisphere's half is YANKEE + 12.
Now we come to a very important point in our discussion. Since we are considering the MIKE and YANKEE zones to be a single zone, it follows that the time in MIKE is always the same as that in YANKEE. This is where the International Date Line comes into play, for whenever this line is crossed, whether from east to west or from west to east, the day must change. Since we have already established that there is a 1-hour difference between each of the 24 time zones, it is clear that there is always a situation where it is a day earlier or later in one part of the world than it is in another.
A final point of discussion involving the International Date Line and zones MIKE and YANKEE is the "gaining" or "losing" of a day as the line is crossed. This is not a problem. "Gaining" or "losing" is nothing more than a question of semantics and should not be used in time conversion conversation.
The formula for determining whether to add or subtract one day from the current day at the time of departing one hemisphere for another is:
When you cross the International Date Line, apply the sign of the departed hemisphere. For example, to go from the MIKE zone into the YANKEE zone, subtract one day. MIKE is in the eastern (or the "-") hemisphere. To go from the YANKEE zone into the MIKE zone, add one day. YANKEE is in the western (or the "+") hemisphere. From "-" to "+," subtract; from "+" to "-," add. Another method is simply to remember to add a day when crossing the line westbound and subtract a day when crossing eastbound.