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


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`

cgf


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