[issue44831] Inconsistency between datetime.now() and datetime.fromtimestamp(time.time(), None)

[Paul Ganssle]

Paul Ganssle <p.ganssle at gmail.com> added the comment:

I think this is a rounding issue. `time.time()` returns an epoch timestamp as a float and at the current epoch time, floats are spaced ~500ns apart.

`datetime.datetime.now` does a floor division when rounding: https://github.com/python/cpython/blob/8bdf12e99a3dc7ada5f85bba79c2a9eb9931f5b0/Modules/_datetimemodule.c#L5056

`datetime.fromtimestamp` uses the standard banker's round (round above half, tie goes to the nearest even number): https://github.com/python/cpython/blob/8bdf12e99a3dc7ada5f85bba79c2a9eb9931f5b0/Modules/_datetimemodule.c#L5038-L5039

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.

For now I'm going to say that we should target 3.11 on this, since it will change an existing observable behavior for at least one of these functions in a way that isn't necessarily going from "obviously wrong" to "obviously right", so I think we should be cautious and not change this in a patch release.

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versions: +Python 3.11 -Python 3.6, Python 3.8, Python 3.9

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