There are 25 integer world time zones from -12 through 0 (GMT) to +12. Each one is 15° of longitude as measured East and West from the Prime Meridian at Greenwich, England.
Each of the standard time zones is measured relative to the Greenwich Meridian. Time zones have both civilian designations and military designations. Civilian ones use typically three letter abbreviations (e.g. EST for Eastern Time) for most time zones. Military designations use each letter of the alphabet (except 'J', which is not used) and are known by their phonetic equivalent.
A - M are for time zones to the east of the Prime Meridian at Greenwich.
N - Y are for time zones to the West of Greenwich.
Zulu Time (Z) is Greenwich Mean Time (GMT).