[New-bugs-announce] [issue29346] datetime.utcfromtimestamp() returns strange result for very large values

Eli Collins report at bugs.python.org
Sun Jan 22 12:47:34 EST 2017

New submission from Eli Collins:

I've found an odd behavior when passing very large values to ``datetime.datetime.utcfromtimestamp()`` and ``.fromtimestamp()`` under python 3.6.

Under python 3.5, ``utcfromtimestamp(1<<40)`` would throw a ValueError that the year was out of range.  Under python 3.6, this now returns a datetime in year 36812 (which seems reasonable given the input).  

The unexpected behavior occurs when increasing the bits passed:   ``utcfromtimestamp(1<<41)`` returns a datetime with a *smaller* year (6118).  This pattern proceeds as the bits are increased, with the years increasing & then wrapping around again, up to the point where it exceeds time_t (at that point, python 3.6 throws the same OSError as 3.5).

It looks to me like 3.6 dropped a bounds check somewhere, and is now truncating high bits off the resulting year?


Attached is the "dump_timestamp_output.py" script that I was using to examine boundary behavior of utctimestamp() when I found this bug.

System was running Linux Mint 18.1 x86_64, using the python 3.6.0 build from https://launchpad.net/~fkrull/+archive/ubuntu/deadsnakes (ubuntu's python 3.6.0 build also shows this behavior).

components: Library (Lib)
files: dump_timestamp_output.py
messages: 286021
nosy: Eli Collins
priority: normal
severity: normal
status: open
title: datetime.utcfromtimestamp() returns strange result for very large values
type: behavior
versions: Python 3.6
Added file: http://bugs.python.org/file46384/dump_timestamp_output.py

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