RFC 1233 in 1989 noted that the signs of the offsets were specified as opposite the common convention (e.g. RFC 733 published in 1977 allowed using military time zones in the Date: field of emails. The British used a system of lettered zones, which was likely the direct influence. The note states 'This method is based on the systems in use in the Armed Forces of these countries and the United States'. The standard was first distributed by NATO as a note in 1950. Sandford Fleming devised a system assigning the letters A-Y excluding J to 1-hour time zones, which may have been the inspiration for the system.

The ACP 121 standard actually refers to Greenwich Mean Time (GMT) as the base time standard, but UTC has superseded GMT as a more precise time standard, so the time offsets are commonly understood as UTC. The letter 'Z' ('Zulu') indicates Coordinated Universal Time (UTC). The letter 'N' is also used to designate zone -13 this is to provide for a ship in zone -12 keeping Daylight Saving Time. The letter 'L' is also sometimes used to mean 'Local' time, conflicting with its use for the Lima time zone. The letter 'J' ('Juliet'), originally skipped, may be used to indicate the observer's local time. Therefore, adding 5 hours to 0600R produces the time in UTC, 1100Z. For example, the zone description for the Romeo time zone is +5. The numeric zone description or 'plus and minus system' indicates the correction which must be applied to the time as expressed in order to convert to UTC. in zone UTC−5 is written '0600R' and spoken 'zero six hundred Romeo'. The letters are typically used in conjunction with military time. Going west from Greenwich, letters 'November' to 'Yankee' represent zones with negative offsets. Going east from the prime meridian at Greenwich, letters 'Alfa' to 'Mike' (skipping 'J', see below) represent the 12 time zones with positive UTC offsets until reaching the international Date Line. As the ACP 121 standard, the military time zone system is used by the armed forces for Australia, Canada, New Zealand, the United Kingdom, the United States, and many nations in NATO. The military time zone system ensures clear communication in a concise manner, and avoids confusion when coordinating across time zones. The Zulu time zone (Z) is equivalent to Coordinated Universal Time (UTC) and is often referred to as the military time zone. The military time zones are a standardized, uniform set of time zones for expressing time across different regions of the world, named after the NATO phonetic alphabet.

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