Re: POSIX timezone (was Re: date command shows time 20 minutes into future)

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Jan 27 13:16, Christopher Faylor wrote:
> On Fri, Jan 27, 2012 at 05:39:52PM +0100, Corinna Vinschen wrote:
> >On Jan 27 17:02, Corinna Vinschen wrote:
> >> On Jan 27 14:54, Cliff Hones wrote:
> >> > I think the CEST comes from Windows. If you don't have TZ set,
> >> > I think Cygwin turns the timezone names Windows provides into
> >> > abbreviated names by taking the leading letters.
> >> > 
> >> > So Windows "Central European Standard Time" => CEST
> >> > and "Central European Daylight Time" => CEDT
> >> > 
> >> > I've never liked this - arguably Windows is wrong to use non-standard
> >> > naming for the timezones.  It's even worse for us in the UK - we get
> >> > GMTST and GMTDT - ugh.  [UK may be a little unusual, but perfectly
> >> > reasonable in using GMT and BST.]
> >> 
> >> Uh, right.  Thanks for reminding me.  The problem is of course that we
> >> only have this information source, if the environment variable TZ isn't
> >> set.  Worse, the Windows timezone name is potentially language dependent.
> >> Therefore a simple translation table is not sufficient.  It would require
> >> some registry scanning.  Setting TZ is much simpler.
> >
> >Does anybody here think it would be a good idea if Cygwin generates a
> >valid TZ setting if TZ isn't set in the environment when started from a
> >native process?
> >
> >There is a table from Windows timezone key names as used in the registry
> >to TZIDs as used by POSIX on unicode.org(*).  The mechanism in pseudo
> >code would look like this:
> >
> >- TZ set?  All is well.
> >- Otherwise checkout HKLM/SYSTEM/CCS/Control/TimeZoneInformation
> >  - Does value TimeZoneKeyName exist?
> >    Yes (Vista and later) -> Use as keyname
> >    No (pre-Vista) -> x = StandardName
> >    - Iterate over keys under HKLM/Software/MSFT/Win NT/CV/TimeZones
> >    - If x[0] == '@'  test if x == MUI_Std
> >      else test if x == Std
> >      if so, keyname = name of containing key
> >- GetLocaleInfo (LOCALE_USER_DEFAULT, LOCALE_SISO3166CTRYNAME, region);
> >- Map keyname and region to TZID according to table from unicode.org.
> >- set TZ=mapped_TZID
> >
> >Advantage: Always having a valid POSIX timezone information
> >Disadvantage: Takes time every time we start up the first Cygwin process
> >in a process tree.  Not a lot on Vista and later, though.
> 
> I think it would be better to have a program in the distro which could
> set the timezone from Windows given the above.  It could be run when
> needed in a startup script, avoiding the need for Cygwin programs to
> incur a potentially unneeded performance penalty if TZ is not set.
> 
> export TZ=`wintz`
> setenv TZ `wintz`

Good idea.  I try to come up with something over the weekend.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat