In order to compare your local time with any other spot on the globe, you need to know about Universal Time (UT).
Why do we have UT?
When it’s noon on one side of the world, it’s midnight on the other side. In order to avoid confusion, scientists in the 1800s developed Greenwich Mean Time (GMT). It’s based on the time in Greenwich, England, which is at 0° longitude. There’s nothing magic about Greenwich--that’s simply the place where cartographers happened to put 0° longitude! Greenwich Mean Time is now called Universal Time.
UT is based on a 24-hour format. When it’s midnight in Greenwich, it’s 0000 UT. 6 a.m. in Greenwich is 0600 UT, and noon is 1200 UT. If it’s 3:00 p.m. there, that would be 1500 UT, and so on. You don’t need to use “a.m.” or “p.m.” or the colons at all.
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When using UT time, why don’t you need to use AM and PM?
Click the button to check your answer. 
How do I change my local time to UT?
The world is divided into time zones. Each zone covers about 15° longitude. These zones can be looked up in a table or on a map (see below). Based on your time zone, you either add or subtract a certain number of hours to your local time to get UT. Sometimes you have to add a day (if you go over 2400) or subtract a day (if you go below 0000). To go from UT to your local time, just work backwards.
Be careful in summer! If daylight savings time is in effect, then you have to subtract an extra hour to get the correct UT.
Let’s see how this works.
If it’s 2:00 a.m. on January 15th in Greenwich, that would be 0200 UT. What would be the local time around the world?
Let’s try another one. What would the UT be when it’s 7:00 p.m. local time (1900 hours) on July 10th? Remember, you work backwards going from local time to UT.
Now, let’s convert
your time to universal time!
Use the World
Map Time Zone Map map to find the time zone in which you are living.
The number on each time-zone strip shows the “offset”--how many
hours you need to add or subtract from UT to get your local time.
For example, if the map shows “+8”, then add 0800 to UT to get your local time in hours, or subtract 0800 from your local time to get UT. If you’re on Daylight Savings Time right now, be sure to add another hour to get your local time or subtract another hour to get UT.
Here is the offset for a few selected countries – the ones whose names begin with “U”:
| Uganda | +3 hours |
| Ukraine | +2 hours (Local summer +3 hours) |
| United Arab Emirates | +4 hours |
| United Kingdom | 0 hours (Local summer +1 hours) |
| Uruguay | -3 hours |
| USA Eastern | -5 hours (Local summer -4 hours) |
| USA Indiana East | -5 hours |
| USA Central | -6 hours (Local summer -5 hours) |
| USA Mountain | -7 hours (Local summer -6 hours) |
| USA Arizona | -7 hours |
| USA Pacific | -8 hours (Local summer -7 hours) |
| USA Alaska | -9 hours (Local summer -8 hours) |
| USA Aleutian | -10 hours |
| USA Hawaii | -10 hours |
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