[TZInfo-users] GMT timezones

Philip Ross phil.ross at gmail.com
Mon Dec 20 09:16:10 EST 2010


Hi Mark,

On 20 December 2010 02:52, Mark Pentland <mark.pent at gmail.com> wrote:
> In not sure if i'm missing something but it appears the GMT +- timezones are
> inverted.

This is the correct behaviour. TZInfo uses data from the tz database
(http://www.twinsun.com/tz/tz-link.htm), which defines the Etc/GMT+-x
zones in a POSIX-compliant (and confusing) manner. The documentation
for the tz database Etc zones states the following:

# We use POSIX-style signs in the Zone names and the output abbreviations,
# even though this is the opposite of what many people expect.
# POSIX has positive signs west of Greenwich, but many people expect
# positive signs east of Greenwich.  For example, TZ='Etc/GMT+4' uses
# the abbreviation "GMT+4" and corresponds to 4 hours behind UTC
# (i.e. west of Greenwich) even though many people would expect it to
# mean 4 hours ahead of UTC (i.e. east of Greenwich).

> I'm in Brisbane/Australia (GMT + 10), so i'm expecting it to behave similar
> to Etc/GMT+10

> Is my assumption incorrect?

Following the POSIX definition, Etc/GMT+10 is a timezone that is fixed
at 10 hours behind GMT. For 10 hours ahead of GMT, you'll need
Etc/GMT-10.

Unless you have a specific need for a timezone that has a fixed GMT
offset (and doesn't adjust for daylight savings), you would be better
off using the geographical zones (such as Australia/Brisbane).

Kind regards,

Phil

-- 
Phil Ross
http://tzinfo.rubyforge.org/ -- DST-aware timezone library for Ruby


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