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gh-88994: Change `datetime.now` to half-even rounding for consistency with `datetime.fromtimestamp`. by jhohm · Pull Request #134258 · python/cpython · GitHub
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gh-88994: Change datetime.now to half-even rounding for consistency with datetime.fromtimestamp. #134258

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Merged
merged 5 commits into from
May 19, 2025
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gh-88994: Change datetime.now to half-even rounding for consistency with datetime.fromtimestamp. #134258

merged 5 commits into from
May 19, 2025

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@jhohm
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@jhohm jhohm commented May 19, 2025 •
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Resolves #88994 where, on platforms with nanosecond resolution system clocks (such as Windows), the truncation within datetime.now could cause time to appear to go backwards by 1 microsecond between datetime.datetime.fromtimestamp(time.time(), None) and datetime.datetime.now().

As suggested by @pganssle, not adding a test that would probably be flaky since it would involve comparing the results of multiple reads of the system clock. Verified manually that the following quickly fails the assert on Windows but passes with this commit:

import datetime
import time

def test():
    start = time.time()
    end = datetime.datetime.now()
    start = datetime.datetime.fromtimestamp(start, None)
    assert end >= start, f'{end=} < {start=}'

for i in range(10000000):
    test()

@jhohm jhohm requested review from abalkin and pganssle as code owners May 19, 2025 17:46
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python-cla-bot bot commented May 19, 2025 •
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bedevere-app bot commented May 19, 2025

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

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Would you mind to sign the CLA?

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You should add a NEWS entry to document your change, see https://devguide.python.org/ which explains how to add one.

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vstinner approved these changes May 19, 2025
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LGTM

cc @pganssle

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@jhohm If you are not already in the ACKS file, do you want to add yourself?

Misc/NEWS.d/next/Library/2025-05-19-18-12-42.gh-issue-88994.7avvVu.rst Outdated
@@ -0,0 +1,2 @@
Change :func:`datetime.datetime.now` to half-even rounding for
consistency with :func:`datetime.datetime.fromtimestamp`.
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If you want you can also add "Patch by (Your name however you want it rendered)." here.

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StanFromIreland commented May 19, 2025 •
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Verified manually that the following quickly fails the assert on Windows but passes with this commit:

I am unable to reproduce such success on Linux?

StanFromIreland
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@jhohm @StanFromIreland
Conform to PEP 7 on changed line.
15a97fd 
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
@vstinner vstinner merged commit 4709417 into python:main May 19, 2025
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Merged, thank you for your contribution!

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StanFromIreland commented May 19, 2025 •
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@vstinner out of curiosity does it work for you, it's hard to believe it's somehow windows only.

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@vstinner out of curiosity does it work for you, it's hard to believe it's somehow windows only.

Using a debug build, I failed to reproduce the issue. I trust the code, I know what it does :-)

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jhohm commented May 19, 2025

Huh; I can still reproduce the problem but only on a release Linux build. I'm investigating to see if there's something else I can do to ensure consistency there.

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start = time.time()
end = datetime.datetime.now()
start = datetime.datetime.fromtimestamp(start, None)

time.time() uses float which is not great in terms of accuracy. Using time.time_ns() would be better. Sadly, there is no "fromtimestampns()" function.

@jhohm jhohm deleted the gh-88994 branch May 20, 2025 13:25
Pranjal095 pushed a commit to Pranjal095/cpython that referenced this pull request Jul 12, 2025
@jhohm @Pranjal095
pythongh-88994: Change datetime.datetime.now to half-even rounding (p…
535b369 
…ython#134258)

Change `datetime.datetime.now` to half-even rounding
for consistency with `datetime.fromtimestamp`.
taegyunkim pushed a commit to taegyunkim/cpython that referenced this pull request Aug 4, 2025
@jhohm @taegyunkim
pythongh-88994: Change datetime.datetime.now to half-even rounding (p…
85ef9f1 
…ython#134258)

Change `datetime.datetime.now` to half-even rounding
for consistency with `datetime.fromtimestamp`.
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@StanFromIreland StanFromIreland StanFromIreland left review comments

@vstinner vstinner vstinner approved these changes

@cmaloney cmaloney cmaloney approved these changes

@pganssle pganssle pganssle approved these changes

@abalkin abalkin Awaiting requested review from abalkin abalkin is a code owner

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

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@jhohm @vstinner @StanFromIreland @cmaloney @pganssle