While investigatinghttp://bugs.python.org/issue30302 it was discovered that the Python implementation of the datetime module was not being tested correctly.The datetimetester.py was supposed to execute tests twice, once for the _Fast implementation (i.e. C extension code in _datetimemodule.c) and once for the _Pure implementation (i.e the Python code).The fix is contained in the following two commits: -https://github.com/python/cpython/pull/1493/commits/08e7548f56838fca43b488cefe51de4bdd600f3d -https://github.com/python/cpython/pull/1493/commits/94d5a4e4d33a1d14a2bb1be8fbff5e1e4cd2b7a6The bug does not effect Python 2.7 as the C version of the datetime module had not been introduced back then. However, as fas as I can tell, the fix needs to be applied to all Python 3.x branches.