May 072010

As I was flying to Japan today, I crossed 11 time zones. When I was in the Navy, I had to really understand time zones because I was regularly working with other commands across many different timezones. Today’s posting is intended to teach you a little more about time zones.
There are 25 world time zones. Each time zone is centered upon every 15 degrees of longitude (7-1/2 degrees on each side of the 15 degree increment) except for the two time zones on either side of the international date line. Those are each only 7-1/2 degrees wide.
Each time zone is represented numerically by a number and a plus or minus sign, such as -5. That number indicates what you need to add to GMT in order to calculate the time in that time zone. In the example of -5 (which happens to be the time zone for EST, or the east coast of the United States in the summer time), we need to add -5 hours to the local time in GMT to determine the local time in the -5 time zone. GMT is centered on the prime meridian, or 0 degrees longitude. It is twelve hours from the international dateline.
The time zones are also represented in the US military by a letter. This works out great since there are 25 time zones and there are 26 letters (want to guess which letter is not used?). GMT (also called UTC for Universal Time Code) is represented by the letter Z, or the word ZULU. The first zone to the west of GMT is labeled N. But it is 1 hour before GMT, it is also labeled -1. To put it all together, we usually call that time -1N (and we usually use the phonetic spelling of N, which is NOVEMBER). So if you were talking about that time zone, you would refer to it as “minus one November”. The next time zone to the west is mike, and it is -2. The first time zone to the east of GMT is A (or alpha) and is labeled +1. And so on. See the table below for the list of all time zones and it’s letter designation.

Abbreviation Full name Time zone Start Long. Center Long. End Long.
Z Zulu Time Zone UTC -7.5 0 7.5
Y Yankee Time Zone UTC – 12 hours -180 -176.25 -172.5
X X-ray Time Zone UTC – 11 hours -172.5 -165 -157.5
W Whiskey Time Zone UTC – 10 hours -157.5 -150 -142.5
V Victor Time Zone UTC – 9 hours -142.5 -135 -127.5
U Uniform Time Zone UTC – 8 hours -127.5 -120 -112.5
T Tango Time Zone UTC – 7 hours -112.5 -105 -097.5
S Sierra Time Zone UTC – 6 hours -097.5 -090 -082.5
R Romeo Time Zone UTC – 5 hours -082.5 -075 -067.5
Q Quebec Time Zone UTC – 4 hours -067.5 -060 -052.5
P Papa Time Zone UTC – 3 hours -052.5 -045 -037.5
O Oscar Time Zone UTC – 2 hours -037.5 -030 -022.5
N November Time Zone UTC – 1 hour -022.5 -015 -007.5
A Alpha Time Zone UTC + 1 hour 007.5 015 022.5
B Bravo Time Zone UTC + 2 hours 022.5 030 037.5
C Charlie Time Zone UTC + 3 hours 037.5 045 052.5
D Delta Time Zone UTC + 4 hours 052.5 060 067.5
E Echo Time Zone UTC + 5 hours 067.5 075 082.5
F Foxtrot Time Zone UTC + 6 hours 082.5 090 097.5
G Golf Time Zone UTC + 7 hours 097.5 105 112.5
H Hotel Time Zone UTC + 8 hours 112.5 120 127.5
I India Time Zone UTC + 9 hours 127.5 135 142.5
K Kilo Time Zone UTC + 10 hours 142.5 150 157.5
L Lima Time Zone UTC + 11 hours 157.5 165 172.5
M Mike Time Zone UTC + 12 hours 172.5 176.25 180

So, what happens when daylight savings kicks in? Daylight savings (also known as “Summer Time”) affects the zone descriptions. Assume one day I am on standard time and I have a particular zone description (say, -5R). When daylight savings kicks in, I will then have a different description (in this case, -4Q).
The time zones do not follow the 15 degree +/- 7-1/2 longitude lines exactly. Sometimes the lines are adjusted for political reasons or convenience.
What’s more, there are some areas that are 1/2 hour ahead/behind the nearest time zone instead of the usual hour and there are even some areas that follow quarter-hour differences.

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