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

Brian Inglis
Fri Jun 11 17:33:05 GMT 2021

On 2021-06-10 13:50, Ken Brown via Cygwin wrote:
> On 6/10/2021 2:31 PM, Brian Inglis wrote:
>> 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
> Right, and I'm agreeing with Bruno (in the message cited above) that the 
> default startup scripts should stop doing that.
>> to get the correct behaviour from Cygwin DLL and programs,
> Can you be more specific?  What goes wrong if TZ is not set?  I haven't 
> seen any POSIX or Linux documentation that says it should be set, and 
> I've just checked on two different Linux distros that it's not set by 
> default.

I would expect that date, ls, etc. would output UTC, or perhaps PST or 
EST, depending on tzdata builds of Factory (tz -00/unset) and posixrules 
(Cygwin PST, Debian EST) and use during system setup and startup, unless 
/etc/timezone and/or /etc/localtime are set, and used during startup, 
often by systemd, or login by profiles.

How do you set or get useful local time on your systems?

On Debian I "sudo dpkg-reconfigure tzdata" to set America/Edmonton after 
install, locale-gen a few en_?? locales, and localedef personal local 
customizations to en_CA.

I can't remember what I may have done setting up other distros when I 
tried them.

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

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