STINNER Victor <vstin...@python.org> added the comment:
> Presumably if we change these two to be consistent, this issue will go away.
> I am not entirely sure if anyone is relying on a particular rounding behavior
> for one or both of these, and I'm not sure which one is the right one to
> harmonize on.
We already changed datetime rounding once in Python 3.4.4, see bpo-23517.
commit 511491ade0bb77febb176bc75f049797f0c71ed0
Author: Victor Stinner <victor.stin...@gmail.com>
Date: Fri Sep 18 14:42:05 2015 +0200
Issue #23517: Fix rounding in fromtimestamp() and utcfromtimestamp() methods
of datetime.datetime: microseconds are now rounded to nearest with ties
going
to nearest even integer (ROUND_HALF_EVEN), instead of being rounding towards
zero (ROUND_DOWN). It's important that these methods use the same rounding
mode than datetime.timedelta to keep the property:
(datetime(1970,1,1) + timedelta(seconds=t)) ==
datetime.utcfromtimestamp(t)
It also the rounding mode used by round(float) for example.
Add more unit tests on the rounding mode in test_datetime.
Since that time, I wrote a lot of time in Python/pytime.c to handle various
rounding methods, and handle various time formats. See
Include/cpython/pytime.h, I added documentation at the top recently ;-)
https://github.com/python/cpython/blob/main/Include/cpython/pytime.h
The _datetime module doesn't use _PyTime_t type but time_t to support the
time_t full range (larger than _PyTime_t with 64-bit time_t).
I wrote an article about rounding timestamps:
https://vstinner.github.io/pytime.html
It seems like not all issues have been fixed yet :-)
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