, dateutil library ( , Python 3 +), dateutil.parser.parse, datetime, datetime.datetime.strftime(), - 'YYYY-MM-DD'. -
>>> s = """12/31/1991
... December 10, 1980
... September 25, 1970
... 2005-11-14
... December 1990
... October 12, 2005
... 1993-06-26"""
>>> from dateutil import parser
>>> for i in s.splitlines():
... d = parser.parse(i)
... print(d.strftime("%Y-%m-%d"))
...
1991-12-31
1980-12-10
1970-09-25
2005-11-14
1990-12-10
2005-10-12
1993-06-26
, dateutil.parser.parse -, datetime, ( 'December 1990'), - 1990-12-10, 10 - ).