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

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

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.


Corinna


(*) http://unicode.org/repos/cldr-tmp/trunk/diff/supplemental/zone_tzid.html

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