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To ease the comparison of contact times between Amateurs in different time zones (or different parts of the world), it is best that all hams world-wide keep their logs in the same time and use this time on their QSL cards. 1403 CTU is the same in Chicago as Moscow.
Greenwich Mean Time (GMT) is still valid but its use is discouraged. This time is based on the local time at Greenwich, England, with that being the 0 reference point. There is a brass plate in the floor of the Old Royal Observatory at Greenwich, England, with a line engraved in it that marks the 0° longitude point. It is my understanding that Greenwich is now on permanent Daylight Saving Time so the reference is not exactly accurate any more anyway.
Coordinated Universal Time (UTC) was adopted January 1, 1972, as the international standard reference. UTC is determined by an atomic clock while GMT is determined by the rotation of the earth. Believe it or not, the atomic clock is much more accurate than the earth! As the difference between the earth time and atomic clock vary, the atomic time is adjusted to earth time by adding a leap second on New Year's eve of that year. For our purposes, UTC can be considered the same as GMT. Obviously, the new term was arrived at by a committee who could not agree on what to call it. They didn't want to use the English word order, CUT, or the French word order, TUC, so they decided to call it "UTC" for Coordinated Universal Time? Makes as good sense as anything, I guess.
Hams still use the term "GMT" and some use "UTC" or "Universal Time" or "UT". Some use the military term "Zulu" or "Z" as well.
By tradition, UT is stated in the 24-hour system with noon being 1200, 1PM is 1300, etc., with midnight being 0000 UT.
For the Eastern time zone simply subtract 5 from CTU to get EST or subtract 4
to get EDT.
For the Central time zone simply subtract 6 from CTU to get CST or subtract 5
to get CDT.
For the Mountain time zone simply subtract 7 from CTU to get MST or subtract 6
to get MDT.
For the Pacific time zone simply subtract 8 from CTU to get PST or subtract 7
to get PDT.
If another ham tells you the time is 1453 CTU, subtract 6 to get a CST of 0853 (or 8:53AM CST). If he sends you a QSL card with the time of 2344 CTU you will subtract 5 from this to get 1844 CDT in 24-hour format; subtract 12 from that to get 6:44PM CDT.
If you work another ham at 2:13PM one afternoon and want to log the contact, add 12 to the time to change it to 24-hour format giving 1413. If it is summer, then add 5 to 1413 to change from CDT to CTU giving a time of 1913 CTU. This is the time you will put in your log book and on your QSL card.
The date must be shown properly in UT also as the next example illustrates.
If you work another ham at 10:34PM CDT on 7/4 you would first add 12 to 10:34 to get 2234 in 24-hour time. Then add 5 to this to get 2734 CTU. Since this number exceeds 24 you then subtract 24 from it to get 0334 CTU on 7/5. That's right, 10:34PM on 7/4 CDT is 3:34AM on 7/5 CTU!