Can I create PO with need by date as backdated Date?
Yes, We can.
Need by date is intentionally allowed to be backdated to support the data entry of old documents .Why because Purchasing documents could be entered after the receipt of the goods.
In such cases, it is important to allow the back date on the PO Lines to support those past transactions.
The profile "PO_CHECK_OPEN_PERIOD" controls that the need by date should be in an open purchasing period.
However, like other products in EBS, Purchasing also allows the back dated PO lines.
This is proper functionality.
If there is any specific reason to enforce this, by using form personalization or custom validations can be made to achieve this objective.