jayvdb added a comment.
There is also the very useful https://labix.org/python-dateutil
If we want our own implementation, the core problem is getting the number of days in the month, and there are a few ways to achieve that in python
http://stackoverflow.com/questions/42950/get-last-day-of-the-month-in-python
Once that algorithm is in core, the ability to do month operations is simple.
TASK DETAIL https://phabricator.wikimedia.org/T73124
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