Получите данные согласованно, затем примените приведение / преобразование.
DECLARE @dates TABLE (
DateString VARCHAR(50)
,[Date] DATE
)
INSERT INTO @dates (DateString) SELECT '11/12/2014'
INSERT INTO @dates (DateString) SELECT '12 December 2014'
INSERT INTO @dates (DateString) SELECT '15/12/2014'
UPDATE @dates
SET [Date]=
CONVERT(DATE,
REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(DateString,
' January ','/01/'),
' February ','/02/'),
' March ','/03/'),
' April ','/04/'),
' May ','/05/'),
' June ','/06/'),
' July ','/07/'),
' August ','/08/'),
' September ','/09/'),
' October ','/10/'),
' November ','/11/'),
' December ','/12/')
,103)
SELECT * FROM @dates