def add_one_month(t): """Return a `datetime.date` or `datetime.datetime` (as given) that is one month earlier. Note that the resultant day of the month might change if the following month has fewer days: >>> add_one_month(datetime.date(2010, 1, 31)) datetime.date(2010, 2, 28) """ import datetime one_day = datetime.timedelta(days=1) one_month_later = t + one_day while one_month_later.month == t.month: # advance to start of next month one_month_later += one_day target_month = one_month_later.month while one_month_later.day < t.day: # advance to appropriate day one_month_later += one_day if one_month_later.month != target_month: # gone too far one_month_later -= one_day break return one_month_later def subtract_one_month(t): """Return a `datetime.date` or `datetime.datetime` (as given) that is one month later. Note that the resultant day of the month might change if the following month has fewer days: >>> subtract_one_month(datetime.date(2010, 3, 31)) datetime.date(2010, 2, 28) """ import datetime one_day = datetime.timedelta(days=1) one_month_earlier = t - one_day while one_month_earlier.month == t.month or one_month_earlier.day > t.day: one_month_earlier -= one_day return one_month_earlier