VARCHAR TO Datetime conversion in SSIS

Hello Guru's,

                    I am importing a cloumn (xdate) from excel to OLEDB table. Xdate has eother NULLS or datetime in excel sheet . I am OLEDB source (xlsx file) for which XDate is read as Varchar(255).

While the Xdate in the OLEDB Destination tables is a Datetime. I am using a derived column expression

TRIM(Xdate) == "NULL" ? NULL(DT_DBTIMESTAMP) : (DT_DBTIMESTAMP)Xdate

its throwing me an error...plzz help me out???