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In talking about an upcoming flight, I was explaining that all flight planning is done in the background in Zulu time (UTC). I understand that, and it makes sense, and I know that tickets are only printed for in local time for pax convenience.

It got me thinking, though. There are 26 letters in the English alphabet, and 24 time zones around the Earth. Are all time zones given an alphabetic equivalent, skipping I and O (perhaps) so that while Zulu time is at the prime meridian, is the next time zone east Alpha time, while the next time zone west of it would be Yankee time?

I realize that this is probably tenuous here at Aviation, but it seems reasonable.

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    $\begingroup$ Just an aside, because it doesn't affect the main question - 24 hours does not mean 24 time zones. Google lies when I search "how many time zones", quoting an article form India Today as the first hit. Counts vary but 38 list time zones is a widely accepted answer (excluding daylight saving, which may not be applied uniformly). I don't know if any of these lack aviation facilities that may need to use local time. The tz timezone database contains hundreds, but represents historical changes. $\endgroup$
    – Chris H
    Nov 19 '21 at 13:24
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    $\begingroup$ @ChrisH: Some timezones have offsets of 30 and 45 minutes, not full hours. They will never be in the A-Z times which is a code for hour offsets, not timezones. $\endgroup$
    – mins
    Nov 19 '21 at 13:30
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    $\begingroup$ @mins "there are 24 time zones" is a quote from the question, and what I was addressing. You and I are pointing out something similar but from different directions; your 2nd sentence sum it up nicely $\endgroup$
    – Chris H
    Nov 19 '21 at 13:32
  • $\begingroup$ I used to think it was called "Zulu" because the Prime Meridian passes through Africa. $\endgroup$ Nov 20 '21 at 7:30
  • $\begingroup$ After that got you thinking, what research did you do? Why didn't you Post that here? $\endgroup$ Nov 20 '21 at 21:53
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Yes. From Wikipedia's List of Military Time Zones, a letter is assigned to each time zone, with Alpha representing UTC+1 and running through Yankee, UTC–12. Zulu, of course, is UTC+0. Juliet is not used and "may be used to indicate the observer's local time."

Note that there are twenty-five time zones represented in this system: twelve zones to the left of UTC, twelve to the right, and UTC itself. This system does not cover all time zones in the world, which range from UTC–12 to UTC+14—plus several in-between time zones.

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    $\begingroup$ Notably, this list of military "time zones" is based solely on the offset from UTC - which may not always cleanly correspond with actual time zones used in practice (due to Daylight Saving Time, and some time zones having offsets that are not in hour intervals). $\endgroup$
    – V2Blast
    Nov 19 '21 at 18:44
  • $\begingroup$ Most places that to DST do it by one hour would would just move to an adjacent time zone, but I am aware of one exception. $\endgroup$
    – Jasen
    Nov 19 '21 at 23:21
  • $\begingroup$ The big question is why was Juliet skipped? $\endgroup$ Nov 20 '21 at 21:06
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    $\begingroup$ @RockPaperLz-MaskitorCasket No, the big question is why aren't they laid out contiguously? Why are M and N on opposite sides of the Earth, same with Y and Z. $\endgroup$
    – Arthur
    Nov 21 '21 at 12:04
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    $\begingroup$ "this list of military "time zones" is based solely on the offset from UTC - which may not always cleanly correspond with actual time zones used in practice" Cannot emphasize this enough. My main gig is time series data, and I've spent a disturbingly high proportion of my career just untangling time zones. $\endgroup$
    – Jason
    Nov 21 '21 at 22:44
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Zulu for "Z" because it is the Zeroth meridian, passing through the Greenwich observatory, not necessarily because the UTC offset is zero, although this is a consequence.

Yes there is an extension based upon whole hour increments, and yes there are more than 24 time zones since some align to a half hour.

"A" was for "Atlantic Time" and "Y" was for "Yukon" time, in a mnemonic fashion, see RFC724, noting convenience of "H" for Hawaii.

RFC 733 explicitly states "J" is not used. These are but humble RFC's but ratify the convention in the digital age.

It was a US military extension. Whereas the email header +/- convention is more widely used by mail servers. It makes sense that ARPA (& Net) would follow. Aviation usage would be more informal.

Table from RFC 733 Military is taken to mean US military.

zone        = ( ["-"] ( "GMT"               ; Relative to GMT:
                                            ; North American
                 /  "NST" /                 ;  Newfoundland:-3:30
                 /  "AST" / "ADT"           ;  Atlantic: - 4/ - 3
                 /  "EST" / "EDT"           ;  Eastern:  - 5/ - 4
                 /  "CST" / "CDT"           ;  Central:  - 6/ - 5
                 /  "MST" / "MDT"           ;  Mountain: - 7/ - 6
                 /  "PST" / "PDT"           ;  Pacific:  - 8/ - 7
                 /  "YST" / "YDT"           ;  Yukon:    - 9/ - 8
                 /  "HST" / "HDT"           ;  Haw/Ala   -10/ - 9
                 /  "BST" / "BDT"           ;  Bering:   -11/ -10
                    1ALPHA       ))         ; Military: Z = GMT;
                                            ;  A:-1; (J not used)
                                            ;  M:-12; N:+1; Y:+12
            / ( ("+" / "-") 4DIGIT )        ; Local differential
                                            ;  hours/min. (HHMM)

Of course the differential format is predominantly used in email headers.

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  • $\begingroup$ Isn't there also a :45 time zone? Maybe Tibet? $\endgroup$
    – JoelFan
    Nov 20 '21 at 22:20
  • $\begingroup$ @JoelFan Chatham Islands, New Zealand. $\endgroup$ Nov 22 '21 at 2:37
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    $\begingroup$ @JoelFan Nepal uses UTC+05:45 $\endgroup$
    – Jan
    Nov 22 '21 at 9:42
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The German Wikipedia gives also some details about then time zones which are not full hours. However, the topic does not provide a references, so it could be false.

For time zones that differ from UTC by odd multiples of half an hour, an asterisk is added to the letter of the time zone closer to the prime meridian, and a diamond is added for a distance of 45 minutes. Examples are Newfoundland in time zone P* (UTC-03:30) or Afghanistan in time zone D* (UTC+04:30). Exceptions are the M time zones, which are more than 12 hours apart from UTC. For example, the Line Islands have a time difference of 14 hours from UTC (UTC+14:00), but are west of the International Date Line, so the time zone is marked with a cross as M†. The same exception applies to the time zones M♦ (UTC+12:45, Chatham Island) and M* (UTC+13:00, Phoenix Islands and Tonga).

Translated with www.DeepL.com/Translator

Maybe, it should be noted that these "military" time zones are actually used by the NATO and regions like Chatham Island or Tonga simply don't have so much interest for the NATO. Forces which are operating there, most like will use Zulu time.

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  • $\begingroup$ Zulu time is also used in civil aviation. $\endgroup$
    – Vladimir F
    Nov 20 '21 at 0:22
  • $\begingroup$ @VladimirF yes, there is no question about it. Question is what about the other characters A to Y? $\endgroup$ Nov 21 '21 at 9:37
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All time zones are attributed a name from the NATO phonetic alphabet, from A to Z (Z timezone being UTC).

Note that while timezone L is attributed to UTC+11, I have seen it more than once being used to represent local time (L for "local" and not Lima, I assume). While local time is supposedly coded J as per randomhead's answer and source, I have never encountered such.

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    $\begingroup$ This can't be true, because there are more than 24 time zones around the world. $\endgroup$ Nov 19 '21 at 23:03
  • $\begingroup$ Wernfried Domscheid: Would you elaborate about the more than 24 time zones? $\endgroup$
    – Carel
    Nov 21 '21 at 21:08

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