Expression derived coloumn

Question

I have an column in my source table as
Valyyyymmdd   [Valyyyymmdd]
[nvarchar](24) NULL 
=================
20130503
20120403
00000000
20110523
20100715
I want to get the difference with getdate()
so i used the below query in my source
DATEDIFF(DAY, IIF( [Valyyyymmdd] ='00000000', CONVERT(VARCHAR(8), GETDATE(), 112), [Valyyyymmdd]) , getdate()) as SalesStageAging
but i need to get the Valyyyymmdd and do ssis derived column to get the difference in date resulting in int value.
I tryed as below
DATEDIFF("d",[Valyyyymmdd] == "00000000" ? GETDATE() : (DT_DBDATE)(SUBSTRING([Valyyyymmdd],1,4) + "-" + SUBSTRING([Valyyyymmdd],5,2) + "-" + SUBSTRING([Valyyyymmdd],7,2)),GETDATE())
kindly provide me the expression which has to be written in derived column expression
ShanmugaRaj

Answer 1

Note  :
SourceColumn  [Valyyyymmdd]
[nvarchar](24) NULL  
DestinationColumn   [DestColumn] [int] NULL,
After Implementing 
((DT_WSTR,24)DestColumn == "00000000" ? GETDATE() : (DT_DBDATE)(SUBSTRING((DT_WSTR,10)DestColumn,1,4) + "-" + SUBSTRING((DT_WSTR,10)DestColumn,5,2) + "-" + SUBSTRING((DT_WSTR,10)DestColumn,7,2)))
[OLE_DST TargetTable [89]] Error: SSIS Error Code DTS_E_OLEDBERROR.  An OLE DB error has occurred. Error code: 0x80004005.
An OLE DB record is available.  Source: "Microsoft SQL Server Native Client 11.0"  Hresult: 0x80004005  Description: "Invalid character value for cast specification".
[OLE_DST TargetTable [89]] Error: There was an error with OLE_DST TargetTable.Inputs[OLE DB Destination Input].Columns[DestColumn] on OLE_DST TargetTable.Inputs[OLE DB Destination Input]. The column status returned was: "The value could not be converted because
of a potential loss of data.".
[OLE_DST TargetTable [89]] Error: SSIS Error Code DTS_E_INDUCEDTRANSFORMFAILUREONERROR.  The "OLE_DST TargetTable.Inputs[OLE DB Destination Input]" failed because error code 0xC0209077 occurred, and the error row disposition on "OLE_DST TargetTable.Inputs[OLE
DB Destination Input]" specifies failure on error. An error occurred on the specified object of the specified component.  There may be error messages posted before this with more information about the failure.
[SSIS.Pipeline] Error: SSIS Error Code DTS_E_PROCESSINPUTFAILED.  The ProcessInput method on component "OLE_DST TargetTable" (89) failed with error code 0xC0209029 while processing input "OLE DB Destination Input" (102). The identified component returned
an error from the ProcessInput method. The error is specific to the component, but the error is fatal and will cause the Data Flow task to stop running.  There may be error messages posted before this with more information about the failure.
ShanmugaRaj