SQL statement datetime issue

I have a sql statement that i am trying to use to tell me how many days one date is from another this should be either positive or negative number but I cant seem to get the data correct.  I end up with my created field being populated with other date time stamps.  

SELECT     OIBT.ItemCode, OIBT.BatchNum, POR1.ShipDate, POR1.OpenQty, OIBT.InDate, OPOR.CardCode, OPOR.CardName,
                      POR1.ShipDate - OIBT.InDate AS dayslate
FROM         OIBT INNER JOIN
                      OPDN ON OIBT.BaseType = OPDN.ObjType AND OIBT.BaseNum = OPDN.DocNum INNER JOIN
                      PDN1 ON OPDN.DocEntry = PDN1.DocEntry INNER JOIN
                      OPOR INNER JOIN
                      POR1 ON OPOR.DocEntry = POR1.DocEntry ON PDN1.BaseType = OPOR.ObjType AND PDN1.BaseEntry = OPOR.DocEntry AND
                      PDN1.BaseLine = POR1.LineNum
WHERE     (OPOR.U_SIF_PO_Sample = 'Y')


As you can see i take one datetime field and subtract it from another and want that to equal a number not datetime.