timezone GMT+6 versus GMT-6

[Markus G. Kuhn]

Brian Candler wrote on 1997-03-30 20:31 UTC:
> ...
> Zone	Etc/GMT-6	6	-	GMT-6
> ...
> Zone	Etc/GMT+6	-6	-	GMT+6
> 
> It appears that the zone offsets in column 3 have the wrong sign - can you
> check if this is the case?

As usual, if two different conventions are possible, they are both used
in various situations.  Ok, here is the story again:

In the Unix/POSIX world, timezones east of the prime meridian have a
negative offset and those west of Greenwich have a positive offset. This
gives the U.S. positive values and Russia negative values, which might have
seemed reasonable to the U.S. developers in the 1970s ... ;-)

In the Internet and ISO standards (RFC 822, ISO 8601, etc.), it is exactly
the other way round: east of Greenwich is a positive offset (as these
timezones are "ahead of universal time", and west of Greenwich time
zones are denoted by a negative offset ("behind UTC"). See also
<http://www.ft.uni-erlangen.de/~mskuhn/iso-time.html>.

Many people think, the ISO/Internet convention is more intuitive, but the
POSIX standard (TZ syntax) had to be compatible to existing Unix practice.

Therefore, you'll find different signs at different locations.

The third column uses the Internet/ISO convention, the GMT names
use the old Unix convention for strange backward compatibility reasons.
May be, this should be changed, and these entries should be called
UTC instead of GMT anyway today.

Anyway, there should normally never be a need to use the GMT entries
in the Olson timezone package, as there should be an entry for all
locations, and you definitely should use the entry describing your
location in order to get correct automatic DST switches.

Where are you located?  Time zone entries are named after the largest
populated area in a time zone, e.g. America/Denver for U.S.
Mountain Time (now = UTC-06:00 during the summer). Use this entry if
possible instead of the GMT entries, which were only intended as an
emergency fallback.

To test whether your time is set correctly, use both "date" (show local
time) and "date -u" (show universal time).

Markus

-- 
Markus G. Kuhn, Computer Science grad student, Purdue
University, Indiana, USA -- email: kuhn at cs.purdue.edu