"Military" time zones

Dave Cantor Dave at Cantor.mv.com
Sat Jan 10 16:30:14 UTC 2009

On 09-Jan-2009, Paul Eggert wrote:

> msb at vex.net (Mark Brader; please reply accordingly) writes:
> > As you would know, there is a system of "military" time zone
> > designations where zones are identified by a single letter:
> > http://www.timeanddate.com/library/abbreviations/timezones/milit
> > ary/ gives letter equivalents for zones from -12 = Y to +12 = M.

> > I was having an email conversation with Russ Rowlett, who
> > maintains the units-of-measurement site at
> > <http://www.unc.edu/~rowlett/units/>, and we were wondering
> > whether these single-letter designations are actually still used
> > in practice -- and, if so, how time zones above
> > +12 or not on integer hours are represented.  Is there an
> > +official
> > reference for current practice on this anywhere?
> Yes, please see the Combined Communications-Electronics Board
> general communication instructions, dated 2007
> <http://www.jcs.mil/j6/cceb/acps/acp121/ACP121H.pdf>.  These are
> authoritative for the armed forces of Australia, Canada, NZ, the
> UK, and the US.
> Briefly: "N" is also used for UTC+13, "to provide for a ship in
> zone -12 keeping Daylight Saving Time".  There's no official
> provision for UTC+14, but I presume that'd be "O".  For DST, use
> the corresponding next letter (e.g., the eastern-US "R" becomes
> "Q" during DST.)

I'm not 100% sure, but I think this is a change from past 
practice (though that is probably not important any more).
When I was in the U.S. Army in the late 1960's, I am sure we used 
to affix "+1" to the base letter for a zone to indicate that it 
was DST, that is, for the U.S. Eastern time zone "R", during the 
observance of DST, we would write "2200R+1" not 2200Q.
This was based on the zones labeled with the letter designations 
on a chart much like that shown on page 3B-2 of the referenced 

Also note that the usage of 0000 is deprecated in the referenced 
publication, and that it would appear that 2400 (of the earlier 
date) would be the approved way of indicating a timestamp for 
something occurring exactly at midnight.  This is the opposite of 
the current internet practice.   

(We should try, I believe, try to accommodate time stamps of 2400 
and be able to specify 24 as a valid hour in time zone rules.   
Times after 2400, however belong to the new day and should be 
written, e.g., as 000015 for 15 seconds after midnight.)

> As for non-integer hours, the instructions simply say: "For time
> midway between zones, both letters are used."

I read that as meaning a time exactly halfway between two integer 
zones.   I don't think it's meant to account for a time that's, 
e.g., 8:45 ahead of UTC.   

One more point.  The hour offsets of the lettered zones with 
respect to UTC are the opposite of the usual internet usage.
"R" for the U.S. Eastern time zone, means +5, but the normal 
internet indication for that zone is -0500.   

Dave C.
Dave Cantor
Groton, CT  06340-3731      Dave at Cantor.mv.com

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