Greenwich Mean Time

Formerly the standard for worldwide time, now superceeded by UTC. GMT and UTC are really the same, unless you're worried about fractions of a second. I think they just wanted a cooler acronym, and then came up with some science stuff to justify it.

                Standard  Daylight Savings
                  Time         Time
    Atlantic
    Eastern        EST          EDT
    Central        CST          CDT
    Mountain       MST          MDT
    Pacific        PST          PDT
    Alaska
    Hawaii

Standard time: 

    GMT     ATL  EST  CST  MST  PST  ALA  HAW
difference   -4   -5   -6   -7   -8   -9  -10
    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)

    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)

Mean solar time of the meridian (0° longitude) at Greenwich, England, from which other time zones are calculated. It was originally established within the United Kingdom to regularize railway timetables nationally, and was later adopted internationally.

Coordinated Universal Time (UTC) is the modern standard, and takes into account leap seconds. In popular usage, the two terms are synonymous.