The purchase order delivery date is controlled potentially from 3 places. First is the purchasing info record, there should be one record for each plant that the material is purchased. The info record planned delivery time field controls the date in the PO this field is populated based on the material master planned delivery time field (MRP view. Remember this value is also plant specific, so if a material is not set up completely (all views maintained)at all plants, that may be why you do not get any date. Finally, there is a field on the personal setting of the P.O. create transaction where you could set a specific date, normally this is blank.
Hope this helps