jayvdb added a comment.
Some different algorithms can be found here
http://code.activestate.com/recipes/577274-subtract-or-add-a-month-to-a-date...
TASK DETAIL https://phabricator.wikimedia.org/T73124
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To: jayvdb Cc: pywikipedia-bugs, Legoktm, jayvdb, XZise, Multichill, Xqt