Time zone letter abbreviations
The basis for this is Z - Zulu time - Zero meridian time - the same as UTC
It's a convention to assign letters to time zones, where Z= Zero meridian, and Zulu is the word that represents the letter Z, when it's used in communication. It is sometimes used in the US Military and NATO in conjunction with 24 hour clocks, and is also popular to use in movies to reference time.
Other letters/words are used for other time zones than UTC, most based on the right or reverse order of the Alphabet. Note that 'J' is skipped ('J' - Juliet refers to current local time of the observer). There are 25 time zones defined here, but 26 letters in the English alphabet.