> On Nov 18, 2016, at 7:04 AM, Serhiy Storchaka <report@bugs.python.org> wrote:
> 
> Yet one doubtful detail is that the fold bit is added to the hour bit in datetime.time, but to the month field in datetime.datetime.

The reason behind this choice is documented in PEP 495: "We picked these bytes because they are the only bytes that are checked by the current unpickle code. Thus loading post-PEP fold=1 pickles in a pre-PEP Python will result in an exception rather than an instance with out of range components."

https://www.python.org/dev/peps/pep-0495/#pickles