Understanding the Date/Time Stamps

Universal Time
Weather forecasts and observations are always made with respect to time. By convention, meteorologists use just one time zone: Universal Time, Coordinated (UTC). They also use the twenty four hour clock (where 0000 = midnight UTC).

The date/time stamp on each forecast image represents the time at which the forecast is valid, measured in UTC. This date/time stamp has the following format:

yy mm dd / hh 00V fff

Where:
yy = year that forecast is valid
mm = month that forecast is valid
dd = day that forecast is valid
hh = hour that forecast is valid (UTC)
fff = forecast projection in hours
(As the forecast projection increases, the forecast accuracy generally decreases.)

For example, a forecast with the following date/time stamp:

19991101/0000V048

is a 48-hour forecast valid 0000 UTC November 1, 1999. This is equivalent to 7:00 PM Eastern Standard Time on October 31. If you do not understand why, keep reading....
Time Zones
To convert to local time, we must know the difference between the local time zone and UTC. The time zones and conversions for North America and the Pacific are: 

  Zone   |      Standard Time          |  Daylight Savings Time 
-------- | --------------------------- | ----------------------------
         | Abbreviation    Conversion  | Abbreviation  Conversion
Atlantic |                UTC - 4 hrs  |              UTC - 3 hrs
Eastern  |    EST         UTC - 5 hrs  |    EDT       UTC - 4 hrs
Central  |    CST         UTC - 6 hrs  |    CDT       UTC - 5 hrs
Mountain |    MST         UTC - 7 hrs  |    MDT       UTC - 6 hrs
Pacific  |    PST         UTC - 8 hrs  |    PDT       UTC - 7 hrs
Alaska   |                UTC - 9 hrs  |              UTC - 8 hrs
Hawaii   |                UTC - 10 hrs |              UTC - 9 hrs
Standard Time Conversion Table 
    UTC     ATL  EST  CST  MST  PST  ALA  HAW
    00       8P*  7P*  6P*  5P*  4P*  3P*  2P*
    01       9P*  8P*  7P*  6P*  5P*  4P*  3P*
    02      10P*  9P*  8P*  7P*  6P*  5P*  4P*
    03      11P* 10P*  9P*  8P*  7P*  6P*  5P*
    04      12A  11P* 10P*  9P*  8P*  7P*  6P*
    05       1A  12A  11P* 10P*  9P*  8P*  7P*
    06       2A   1A  12A  11P* 10P*  9P*  8P*
    07       3A   2A   1A  12A  11P* 10P*  9P*
    08       4A   3A   2A   1A  12A  11P* 10P*
    09       5A   4A   3A   2A   1A  12A  11P*
    10       6A   5A   4A   3A   2A   1A  12A
    11       7A   6A   5A   4A   3A   2A   1A
    12       8A   7A   6A   5A   4A   3A   2A
    13       9A   8A   7A   6A   5A   4A   3A
    14      10A   9A   8A   7A   6A   5A   4A
    15      11A  10A   9A   8A   7A   6A   5A
    16      12P  11A  10A   9A   8A   7A   6A
    17       1P  12P  11A  10A   9A   8A   7A
    18       2P   1P  12P  11A  10A   9A   8A
    19       3P   2P   1P  12P  11A  10A   9A
    20       4P   3P   2P   1P  12P  11A  10A
    21       5P   4P   3P   2P   1P  12P  11A
    22       6P   5P   4P   3P   2P   1P  12P
    23       7P   6P   5P   4P   3P   2P   1P

* represents previous day
  (i.e. 03 hours 23 January 93 is 8PM-MST 22 January)
Daylight Savings Time Conversion Table
Note: Not all states observe Daylight Savings Time. For those that do not, use the standard time table all year. 

    GMT     EDT  CDT  MDT  PDT
difference   -4   -5   -6   -7
    00       8P*  7P*  6P*  5P*
    01       9P*  8P*  7P*  6P*
    02      10P*  9P*  8P*  7P*
    03      11P* 10P*  9P*  8P*
    04      12A  11P* 10P*  9P*
    05       1A  12A  11P* 10P*
    06       2A   1A  12A  11P*
    07       3A   2A   1A  12A
    08       4A   3A   2A   1A
    09       5A   4A   3A   2A
    10       6A   5A   4A   3A
    11       7A   6A   5A   4A
    12       8A   7A   6A   5A
    13       9A   8A   7A   6A
    14      10A   9A   8A   7A
    15      11A  10A   9A   8A
    16      12P  11A  10A   9A
    17       1P  12P  11A  10A
    18       2P   1P  12P  11A
    19       3P   2P   1P  12P
    20       4P   3P   2P   1P
    21       5P   4P   3P   2P
    22       6P   5P   4P   3P
    23       7P   6P   5P   4P

* represents previous day
  (i.e. 03 hours 23 June is 9PM-MDT 22 June)