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Subject:	Re: Python for Windows reports wrong local time when run under Cygwin
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From:	Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Ken Brown <kbrown AT cornell DOT edu>
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On 6/11/2021 1:33 PM, Brian Inglis wrote:
> 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 https://bugs.python.org/issue44352.
>>>>>>>
>>>>>>> See Keith Thompson subthread and my reply with suggested fix:
>>>>>>>
>>>>>>> https://cygwin.com/pipermail/cygwin/2021-June/248692.html
>>>>>>>
>>>>>>> Windows does not recognize zoneinfo time zone identifiers in TZ only
>>>>>>> base format POSIX TZ strings with three alphabetic character identifiers:
>>>>>>>
>>>>>>> https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/tzset?view=msvc-160 
>>>>>>>
>>>>>>>
>>>>>>> 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:
>>>>>>>
>>>>>>> https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#bottom 
>>>>>>>
>>>>>>>
>>>>>>> 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:
>>>>>
>>>>>      https://docs.microsoft.com/en-us/cpp/c-runtime-library/compatibility?view=msvc-160 
>>>>>
>>>>>
>>>>> 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:
>>>>>
>>>>>      https://sourceware.org/git/?p=newlib-cygwin.git;f=winsup/cygwin/environ.cc;a=blob 
>>>>>
>>>>>
>>>>> 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:
>>>>
>>>>    https://cygwin.com/pipermail/cygwin/2017-May/232675.html .
>>>>
>>>> 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.

No, you can 'unset TZ' and everything works fine.  Try it yourself.

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

Cygwin takes care of it.  If TZ is unset, Cygwin queries Windows via 
GetTimeZoneInformation.  See tzgetwintzi in 
winsup/cygwin/tzcode/localtime_wrapper.c.

Ken

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