Python for Windows reports wrong local time when run under Cygwin on Europe/Moscow TZ

Brian Inglis
Thu Jun 10 18:31:53 GMT 2021

On 2021-06-10 08:57, Ken Brown via Cygwin wrote:
> On 6/9/2021 10:36 PM, Brian Inglis wrote:
>> On 2021-06-09 16:31, Keith Thompson via Cygwin wrote:
>>> [Sorry if the threading is messed up.  I don't subscribe, so I'm
>>> constructing this message from the web interface.  It should at least
>>> show up under the correct subject.]
>>> Brian Inglis wrote:
>>>> On 2021-06-08 14:03, Mike Kaganski via Cygwin wrote:
>>>>> On 08.06.2021 16:04, L A Walsh wrote:
>>>>>> You might ask on a python list if anyone else has experienced
>>>>>> something similar with python or any other program.  I'm fairly sure
>>>>>> that neither MS nor cygwin design their OS with python in mind and
>>>>>> that it is python that is interacting funny when running under some
>>>>>> merge of both.  Have you asked the python people about this problem?
>>>>>> What did they suggest?
>>>>> FTR: filed
>>>> See Keith Thompson subthread and my reply with suggested fix:
>>>> Windows does not recognize zoneinfo time zone identifiers in TZ only
>>>> base format POSIX TZ strings with three alphabetic character 
>>>> identifiers:
>>>> That assumes US switch date "rules": for all years up to current, or
>>>> just DST, and whether pre- or post-2007 is unstated!
>>>> Otherwise it defaults to regional settings, used by Cygwin to map to
>>>> zoneinfo time zone identifiers, so if Python for Windows could clear TZ
>>>> before it is read by MSVCRT, it should DTRT.
>>>> Windows does not recognize expanded POSIX TZ format strings with <>
>>>> quoted alphanumeric characters, "-", "+", and start and end 
>>>> dates/times:
>>>> which make them usable outside of the US.
>>> Summary: IMHO Cygwin should adapt its default TZ setting to work
>>> with Windows.
>>> The suggestion is to modify Python for Windows so it can deal with
>>> the TZ format used by Cygwin.  I haven't used Python for Windows, but
>>> as far as I know it's unrelated to Cygwin; rather it, like Cygwin, is
>>> intended to work on top of Windows.  I'm not convinced it's appropriate
>>> to ask Python for Windows to make a change purely for the sake of
>>> interoperating with Cygwin, which many PfW users presumably aren't
>>> even using.
>>> I've run into another application that has problems with Cygwin's
>>> settings of $TZ.  It was a internal test application that isn't
>>> going to change its timezone handling just for this problem.
>>> The ideal solution would be for Windows to recognize TZ values like
>>> "America/Los_Angeles", but that's not likely to happen any time soon.
>>> My suggestion, since Cygwin is supposed to interoperate with Windows,
>>> is one of the following:
>>> - Cygwin should avoid setting TZ to a value that Windows doesn't 
>>> recognize
>>>    (if I set TZ=PST8PDT, everything seems to work correctly); OR
>>> - Cygwin shouldn't set TZ at all by default.  (I've updated my
>>>    $HOME/.bash_profile on Cygwin to unset TZ, and Cygwin commands seem
>>>    to work correctly with TZ unset); OR
>>> - Cygwin, when invoking a non-Cygwin executable, should first either
>>>    unset TZ or translate it to a format that Windows will recognize.
>>>    I have no idea how difficult that would be.
>> Impossible to set Windows TZ usefully as it obeys unstated US DST 
>> rules (like posixrules, perhaps 2007+?), and may have limits on hour 
>> offset magnitudes.
>> MS libraries are stuck at POSIX 1996 and C 99 subset compatibility, 
>> but non-standard-conformant including which headers contain definitions:
>> It may be possible to unset TZ when running non-Cygwin programs 
>> (possibly behind a CYGWIN env var setting e.g. winnotz) by adding TZ= 
>> to conv_envvars, and writing new helper functions env_tz_to_posix to 
>> call tzset and env_tz_to_win32 to remove TZ in:
>> What is the opinion on this from both Windows users and Cygwin patchers?
> I'm not convinced it's worth the trouble.  I haven't seen anyone argue 
> that it's useful for Cygwin to set TZ, and I have seen an argument that 
> it's harmful:
> .
> So I prefer Keith's second suggestion:
>  >> - Cygwin shouldn't set TZ at all by default.

It does so in default startup scripts to get the correct behaviour from 
Cygwin DLL and programs, so it's up to users to change the default 
profiles /etc/profile.d/tzset.{,c}sh to disable that, perhaps by putting 
required edits into a local 0p_/zp_ postinstall script.

Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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