Planning Line Document Date wrong in Nav 5.1

Hi Forum, has anyone come across the folowing problem with Document Dates on Job Planning Lines. If I raise a Purchase Invoice with a Posting Date of 19/10/09 and a Document Date of 01/10/09 and post it to a Job. If I look at the Vendor Ledger Entries the Posting Date is still 19/10/09 and the Document Date is still 01/10/09. However when I look at the Job Planning Lines for the Purchase Invoice, the Posting Date, Document Date and Currency Date are all 19/10/09. Can anyone shed light on this please. Thanks Paul