If the data is stored as a datetime type (date, datetime, smalldatetime etc.), you wouldn't need to do any conversion. If it is stored as character (char, varchar etc.), use convert as in:
DECLARE @x VARCHAR(32) = '13/09/2012';
SELECT DATEDIFF(dd,CONVERT(DATETIME,@x,103),GETDATE());Here, I am not specifically converting it to yyyymmdd format. There is no need to because it is best to provide the parameters to DATEDIFF function in its required format - which is datetime.