Military time zones

Military time zones name each whole-hour offset from UTC with a letter of the radio alphabet, so a time can be written with one character and spoken clearly over a radio. The system is set out in ACP 121, a communications standard shared by the armed forces of Australia, Canada, New Zealand, the UK and the US.^[1]

The best-known letter is Z, Zulu, which means UTC. Militaries, ships and aircraft use it as a single shared clock.^{[2, 4]} More in time zones for pilots and UTC vs GMT.

• A to M are east of Greenwich, UTC+1 to UTC+12, skipping J.
• N to Y are west, UTC−1 to UTC−12.
• Z is UTC itself.
• J (Juliet) means the observer's local time, whatever it is.

So New York in winter, five hours behind UTC, is R (Romeo); in summer, four hours behind, it's Q (Quebec).^[1]

A date-time group, or DTG, packs a moment into one string: the day, the time on the 24-hour clock, the zone letter, then the month and year. 091630Z JUL 11 is 16:30 UTC on 9 July 2011.^[3]

• Standard: DDHHMM, letter, month, year, as the generator above shows first.
• With seconds: DDHHMMSS, letter, month, year, run together, for software timestamps.
• Short: DDHHMM and the letter only, for planning within a month.

A DTG in Z needs no conversion for anyone who reads it; one in a local letter tells the reader exactly which offset was meant.

A letter stands for an offset, not a country. Places change letter when their clocks change for daylight saving time, and places on half-hour offsets, such as India (UTC+5:30), have no letter at all.^[5] The lookup above shows a place's letter now and in the other season. The UTC offset calculator gives the letter for any place on any date.

Old email standards allowed single-letter zones in message dates, but they were defined with the signs the wrong way round, so today's standard says to treat them as unknown.^[6] In software, write offsets as numbers, like -05:00, or use Z for UTC.

1. 1.Wikipedia, Military time zone. The letter scheme, J for local time, and the ACP 121 standard that defines it.
2. 2.NIST, How is UTC(NIST) related to UTC, TAI, GMT, GPS time and Zulu time?. Z, or Zulu, meaning UTC in military, nautical and aviation use.
3. 3.Wikipedia, Date-time group. The DDHHMM, letter, month, year format and its variants.
4. 4.FAA, Aeronautical Information Manual, 4-2-12: Time. Aviation using UTC, called Zulu.
5. 5.IANA, Time Zone Database. Places' offsets, which decide the letter they're on at a given moment.
6. 6.IETF, RFC 5322: Internet Message Format, section 4.3. Military zone letters being unreliable in email dates.