[issue36439] Inconsistencies with datetime.fromtimestamp(t) when t < 0

[Dobatymo]

Dobatymo  added the comment:

> I've encountered an issue on anaconda python on windows 10 v1909 which I 
> suspect is related. It looks like no dates in 1970 can be converted to 
> datetime.timestamp():

Yeah... there is more related weirdness going on.

>>> datetime(1970, 1, 3).astimezone(timezone.utc)
datetime.datetime(1970, 1, 2, 16, 0, tzinfo=datetime.timezone.utc)
>>> datetime(1970, 1, 2).astimezone(timezone.utc)
Traceback (most recent call last):
  File "", line 1, in 
OSError: [Errno 22] Invalid argument
>>> datetime(1970, 1, 1, 16, 0, tzinfo=timezone.utc)
datetime.datetime(1970, 1, 1, 16, 0, tzinfo=datetime.timezone.utc)

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[issue36439] Inconsistencies with datetime.fromtimestamp(t) when t < 0

[Paul Anton Letnes]

Paul Anton Letnes  added the comment:

I've encountered an issue on anaconda python on windows 10 v1909 which I 
suspect is related. It looks like no dates in 1970 can be converted to 
datetime.timestamp():

Python 3.8.2 (default, Apr 14 2020, 19:01:40) [MSC v.1916 64 bit (AMD64)]
Type 'copyright', 'credits' or 'license' for more information
IPython 7.13.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import datetime

In [2]: datetime.datetime(1970, 1, 2, 0, 0, 1, 123456).timestamp()
---
OSError   Traceback (most recent call last)
 in 
> 1 datetime.datetime(1970, 1, 2, 0, 0, 1, 123456).timestamp()

OSError: [Errno 22] Invalid argument

In [3]: datetime.datetime(1971, 1, 2, 0, 0, 1, 123456).timestamp()
Out[3]: 31618801.123456

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[issue36439] Inconsistencies with datetime.fromtimestamp(t) when t < 0

[STINNER Victor]

Change by STINNER Victor :


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[issue36439] Inconsistencies with datetime.fromtimestamp(t) when t < 0

[Steve Dower]

Steve Dower  added the comment:

I've emailed some colleagues to see what they can add here.

Ultimately, the Windows CRT is just doing arithmetic, since POSIX time formats 
do not resemble Windows time formats at all, so it's all emulation. So if we 
replaced it with our own calculation I don't think that would be the worst 
possible outcome.

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[issue36439] Inconsistencies with datetime.fromtimestamp(t) when t < 0

[Paul Ganssle]

Paul Ganssle  added the comment:

This has been coming up in a few different contexts lately, so I think it would 
be really good if we could get some sort of fix for it.

One option is to implement our own versions of these APIs for use in Windows, 
but a thought occurred to me recently: we have not investigated the possibility 
of seeing if Microsoft would be willing to either add support for negative 
timestamps in their localtime() or gmtime() implementations or add a new API 
that *does* support negative timestamps. It would also be good to rule out the 
possibility that such APIs already exist but we just don't know about them 
(preliminary googling doesn't show anything, though possibly something can be 
done with the Win32 APIs? Not sure how or if those work in C and how big a lift 
it would be to maintain compatibility if can switch: 
https://docs.microsoft.com/en-us/windows/win32/sysinfo/time-functions?redirectedfrom=MSDN
 ).

Adding Steve Dower to the nosy list in case he can shed some light onto the 
possibility of native support.

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[issue36439] Inconsistencies with datetime.fromtimestamp(t) when t < 0

[Paul Ganssle]

Paul Ganssle  added the comment:

>From the documentation ( 
>https://docs.python.org/3/library/datetime.html#datetime.datetime.fromtimestamp
> ):

> fromtimestamp() may raise OverflowError, if the timestamp is out of the range
> of values supported by the platform C localtime() or gmtime() functions, and
> OSError on localtime() or gmtime() failure. It’s common for this to be
> restricted to years in 1970 through 2038. Note that on non-POSIX systems that
> include leap seconds in their notion of a timestamp, leap seconds are ignored
> by fromtimestamp(), and then it’s possible to have two timestamps differing by
> a second that yield identical datetime objects. See also utcfromtimestamp().

So this is indeed the documented behavior. I agree that it would be good to 
unify the behavior across platforms if possible, but I think this would require 
us to have our own implementation of localtime() and/or gmtime().

That said, it might be possible to implement `fromtimestamp` with some 
equivalent of `datetime(1970, 1, 1) + timedelta(seconds=t)` on Windows when the 
value falls outside the accepted range. We'd probably need some better tests 
under different time zones to make sure that that would be acceptable.

I think it may be a good idea to change the targeted version to 3.8 or 3.9, 
because this is a change to the documented behavior of the function (albeit a 
desirable one that can probably be considered backwards compatible).

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[issue36439] Inconsistencies with datetime.fromtimestamp(t) when t < 0

[Mickaël Schoentgen]

Change by Mickaël Schoentgen :


--
title: OSes inconsistency whith datetime.fromtimestamp(t) when t < 0 -> 
Inconsistencies with datetime.fromtimestamp(t) when t < 0

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