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.

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.