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What is UTC Time?

Weather observations around the world (including radar observations) are always taken with respect to a standard time. By convention, the world's weather communities use a twenty four hour clock, similar to "military" time based on the 0° longitude meridian, also known as the Greenwich meridian. Prior to 1972, this time was called Greenwich Mean Time (GMT) but is now referred to as Coordinated Universal Time or Universal Time Coordinated (UTC). It is a coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM). It is also known a "Z time" or "Zulu Time".

To obtain your local time here in the United States, you need to subtract a certain number of hours from UTC depending on how many time zones you are away from Greenwich (England). The table below shows the standard difference from UTC time to local time.

UTC Diff 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23
AST -4 8P* 9P* 10P* 11P* 12M 1A 2A 3A 4A 5A 6A 7A  8A 9A 10A 11A 12N 1P 2P 3P 4P 5P 6P 7P
EST -5 7P* 8p* 9p* 10p* 11p* 12M 1A 2A 3A 4A 5A 6A 7A 8A 9A 10A 11A 12N 1P 2P 3P 4P 5P 6P
CST -6 6P* 7P* 8P* 9P* 10P* 11P* 12M 1A 2A 3A 4A 5A 6A 7A 8A 9A 10A 11A 12N 1P 2P 3P 4P 5P
MST -7 5P* 6P* 7P* 8P* 9P* 10P* 11P* 12M 1A 2A 3A 4A 5A 6A 7A 8A 9A 10A 11A 12N 1P 2P 3P 4P
PST -8 4P* 5P* 6P* 7P* 8P* 9P* 10P* 11P* 12M 1A 2A 3A 4A 5A 6A 7A 8A 9A 10A 11A 12N 1P 2P 3P
AK -9 3P* 4P* 5P* 6P* 7P* 8P* 9P* 10P* 11P* 12M 1A 2A 3A 4A 5A 6A 7A 8A 9A 10A 11A 12N 1P 2P
HI -10 2P* 3P* 4P* 5P* 6P* 7P* 8P* 9P* 10P* 11P* 12M 1A 2A 3A 4A 5A 6A 7A 8A 9A 10A 11A 12N 1P
Guam +10 10A 11A 12N 1P 2P 3P 4P 5P 6P 7P 8P 9P 10P 11P 12M 1A% 2A% 3A% 4A% 5A% 6A% 7A% 8A% 9A%
Notes: AST - Atlantic Time, AK - Alaska Time, HI - Hawaii Time, * - The previous day, % - The next day

The switch to daylight savings time does not affect UTC. It refers to time on the zero or Greenwich meridian, which is not adjusted to reflect either changes either to or from Daylight Saving Time. However, you need to know what happens during daylight savings time in the United States. In short, the local time is advanced one hour during daylight savings time. As an example, the Eastern Time zone difference from UTC is a -4 hours during daylight savings time rather than -5 hours as it is during standard time.
UTC Diff 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23
EDT -4 8p* 9p* 10p* 11p* 12M 1A 2A 3A 4A 5A 6A 7A 8A 9A 10A 11A 12N 1P 2P 3P 4P 5P 6P 7P
CDT -5 7P* 8P* 9P* 10P* 11P* 12M 1A 2A 3A 4A 5A 6A 7A 8A 9A 10A 11A 12N 1P 2P 3P 4P 5P 6P
MDT -6 6P* 7P* 8P* 9P* 10P* 11P* 12M 1A 2A 3A 4A 5A 6A 7A 8A 9A 10A 11A 12N 1P 2P 3P 4P 5P
PDT -7 5P* 6P* 7P* 8P* 9P* 10P* 11P* 12M 1A 2A 3A 4A 5A 6A 7A 8A 9A 10A 11A 12N 1P 2P 3P 4P

National Weather Service, NOAA
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Silver Spring, MD 20910		 Page Author: Dennis R. Cain
Last Modified: March 12, 2001