This two examples are pretty simple.

On MacOS stftime %Y works as expected, retrieving 4 digits:

Python 3.7.6 (default, Dec 30 2019, 19:38:26)
[Clang 11.0.0 (clang-1100.0.33.16)] on darwin
> datetime.strftime(datetime.now().replace(year=100), "%Y-%m-%d")
'0100-02-03'

But on Linux...:

Python 3.7.6 (default, Jan  3 2020, 23:35:31)
[GCC 8.3.0] on linux
> datetime.strftime(datetime.now().replace(year=100), "%Y-%m-%d")
'100-02-03'


This causes a lot of trouble when storing and then retrieving string dates from any string based storage.