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What is UTC or GMT Time? (Text)


Weather observations around the world (including surface, radar, and other 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.

The switch to daylight saving time does not affect UTC. It refers to time on the zero or Greenwich meridian, which is not adjusted to reflect changes either to or from Daylight Saving Time.

However, you need to know what happens during daylight saving time in the United States. In short, the local time is advanced one hour during daylight saving time. As an example, the Eastern Time zone difference from UTC is -4 hours during daylight saving time rather than -5 hours as it is during standard time.

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

Note:  Credit given to NWS Southern Region Headquarters for the information above.


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Page last modified: Wednesday, 01-Aug-2007 23:57:06 UTC