COMPUTATIONS INVOLVING THE INTERNATIONAL DATE LINE
In our discussions of the International Date Line, we covered two very important points which bear repeating:
1. It is always the same time in zone MIKE as it is in zone YANKEE it is never the same day.
2. When you cross the International Date Line, apply the sign of the departed hemisphere to determine whether to add or to subtract a day. Keep in mind that whenever we cross the line, the day must change.
To illustrate the effect that the International Date Line has upon a DTG, let's assume that we are flying from Tokyo to San Francisco. We begin by listing the facts that we must know about each place:
THE PROBLEM: What will be the local time and date when we land in San Francisco?
To solve this problem, make a graphic chart showing each of the time zones between Tokyo and San Francisco, labeling each zone with its designators. See figure 1-1. (Don't forget to label the International Date Line.) Using our roughly drawn chart, let's fill in the times between Tokyo (-9) and the Date Line. Since we are moving in an easterly direction we add 1 hour upon entering each new time zone.
We have now reached the International Date Line and find that, before crossing the line, the local time is 201100M. We cross the line, departing - 12 and entering + 12. Using our formula for crossing the International Date Line, we apply the sign of the departed hemisphere and subtract 1 day it is now the 19th of April. As stated before, the hour will remain the same in YANKEE (1100Y) as it was when we departed MIKE-only the day changes.
Now let's leave YANKEE and continue adding 1 hour for each new zone. Remember, it is now the 19th, NOT the 20th, as it was when we left Japan.
As we arrive in San Francisco's time zone (UNIFORM), the local time is 191500U. However, this is NOT the answer we are seeking. All we have determined thus far is that when it is the 20th of April at 0800 local time in Tokyo, it is the 19th of April at 1500 local time in San Francisco. We are not finished with the problem until we have added the flying time to the local time in San Francisco. By adding the 13 hours flying time, we find that our arrival time in San Francisco should be 200400U.
COMPUTATIONS INVOLVING DAYLIGHT SAVING TIME (DST)
In computing time conversions, you will frequently encounter problems where one or both of the zones are using DST. Since the purpose of this time modification is to extend the daylight hours (primarily in the summer months), all we have to do is to understand what is done to establish this time.
DST is simply the setting of the clocks in a particular area ahead 1 hour, thus extending the onset of darkness by that margin. Whenever we encounter a problem involving DST, we work the problem according to the methods outlined above, then subtract 1 hour. This will give us the normal time for that zone. If we are asked to solve a time-conversion problem for a time zone using normal time and instructed to give the answer in DST, we work the problem and add 1 hour to obtain the time in DST.
TIME-CONVERSION WORKING AIDS
Most of us have seen charts or maps of the world showing time zones. These are handy tools to have when computing time. Obviously, we can't be expected to carry charts or maps around in our back pocket everywhere we go. The Navy has a 4" x 6" working aid, the time-conversion table. It is small enough to carry in your wallet and is readily available at most field stations. Also, there are many commercially produced materials. Some of these are better than others.
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