Time Zones

Time Conversion must be learned by astrologers to use an ephemeris, which is used to calculate the position of the planets and tell the exact time a specific event will happen in a specific area. At the present time, calculations are done automatically by computer, but there are occasions when you'll want to calculate a time change in your head, such as when you are visiting a web site from another country, or looking at a weather map.

The time listed in an ephemeris (not an astrological calendar) is listed in Greenwich Mean (GMT) Time, for Greenwich meridian in England, which is at zero longitude. This is the time astronomers, navigators, and astrologers have been using for years, and is now used in laboratories, and for the atomic clock.

Universal Time

There is another time which is becoming common, this is called Universal time, (UT. It is also at 0 longitude and is similar to Greenwich time. Sometimes UT time is indicated with a Z, as in 944Z

Longitude

GMT and UT start at zero longitude, as you travel west of Greenwich the time is earlier, east of Greenwich the time is later. For instance New York is west of Greenwich and earlier, Cairo, Egypt is east of Greenwich and later.

GMT and UT both use standard time which is the same all year round. If your country observes Daylight Savings Time (DST), when making a time conversion remember to make an adjustment.

The 24 hour clock

The 24 hour clock does not use AM or PM, it's based on the fact that there are 24 hours in a day. It's mostly used by astronomers, navigators, airlines and the military.

Determine if the time is AM or PM.
When reading the 24 hour clock, time listed lower 12 is AM, while time higher than 12 is PM. Subtract 12 hours from any time over 12 to get PM. For example 18:00 is 6:00 PM, 19:45 is 7:45 PM. For the most part, you don't have to do anything with time below 12. The exception is listed in #4 below with New York as an example.

Converting time zones - Instructions

1. Convert the time to your locality

If you are west of Greenwich, England you must subtract from UT/GMT. If you are east of Greenwich, England you must add.
Examples: New York is 5 hours less than UT. Subtract five hours from UT. If UT is 900, New York is 400.
Cairo, Egypt is 2 more than UT. Add 2 hours to the time. If UT is 900, Cairo is 1100. 

2. Convert to AM or PM

If UT/GMT is higher than 1200, it is PM. If the time is lower than 1200 it is AM. Refer to the section on the 24 hour clock for an example.

3. Adjust for Daylight Savings Time

During DST when going west there is there is one hour less from UT. In the winter, during EST, New York is five hours less than UT, but in the summer, during DST, it is only four hours less. When going east there is one hour extra. Paris is two hours more than UT during Daylight Savings Time, while in the winter, during standard time, it is one hour more.

4. Adjust the day

There are times when the day will change. If the time is 200 UT, that is 2 AM, but in New York it is 9 PM the day before. Subtract 5 hours to convert to New York time. Because the time is subtracted, it is a day earlier and PM in New York.

In the winter, New York, NY is  5 hours less than UT. When it is 16:15 UT it is 11:15 AM in New York. 16:15 - 12:00 = 4:15 PM. Then subtract 5 hours for New York to get 11:15 AM.

If it is 9:00 UT we already know it is AM because 9:00 is less than 12:00. You only need to subtract 5 hours to get New York time which is 4:00 AM.

The Full Moon on 12/15/05 is at 16:15 UT.  Add 2 hours to 16:15 = 18:15. Since it is after 12:00 it is PM. Subtract 12 hours and you get 4:15 PM standard time. The Full Moon on 12/15/05 occurs at 4:15 PM local standard time.

Time Zones Around the World

Remember, the following times are the differences from Universal Time which does not use Daylight Savings Time.

West of Greenwich, England
Subtract the number of hours from Universal Time.

East of Greenwich, England add the hours For example Paris, France is + 1 hour from UT - add one hour.
 

 Difference from UT for Time Zones Around the World

USA

Countries

USA - Central
CST -6
CDT -5

USA - Mountain
MST -7
MDT -8

Alaska Yukon
AKST -9
AKDT -8

Hawaii
AHST -10

Cities Around the World