What is UTC or GMT Time?

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 as "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.

AST - Atlantic, AK - Alaska time, HI - Hawaii time, *The previous day, %The next day
	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

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