From: Jon Hanna (firstname.lastname@example.org)
Date: Wed Aug 20 2003 - 09:34:50 EDT
> > I have no idea whether that's the same conference, but in early 1970's
> >it's also decided that the abbreviation 'GMT' would be deprecated
> >and 'UTC' should be used in its place. ...
There are two subtly different definitions of GMT, one which is synonymous with UTC and one which differs from it; at times by as nearly a second. Hence GMT is ambiguous.
> And to add to confusion, the military also calls it "Zulu time", as in
> 1230 GMT= 1230 UTC = 1230Z.
> Very confusing, especially if you've ever been to South Africa...
> Any one there knows that the Zulu Nation lives at GMT+2 hours!
The abbreviation of Z is used in ISO 8601 and standards, recommendations and specs derived from it, and also in RFC 822.
Indeed the U.S. Military use 25 letters to designate time zones, with A through M skipping J to indicate timezones from +01:00 to +12:00 and N through Y to indicate timezones from -01:00 to -12:00. RFC 822 attempted to copy this but there was an error which resulted in them being used the wrong way around (so A "Alpha time" means -01:00 according to RFC 822 and +01:00 according to the military convention they attempted to copy). The resulting confusion made any attempt to use this scheme in an interoperable way impractical and hence all codes marked obsoleted in RFC 2822, with the advice that one should treat them as indicating +00:00 unless you have out-of-band information about how they are being used. Notably there is no confusion with Z as it means the same time zone whether treated according to the military convention, RFC 822, RFC 2822 or indeed ISO 8601.
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