PO User Exit to Populate Field Validity End During Creation

Hi Gurus,

I'm currently designing a program that will perform auto-deletion of Purchase Orders that have no GRs after the latest Delivery Date of the PO line items. Our client is very strict in managing their purchasing activities hence the requirement. I'm planning to use the Validity End field on the Header level of the PO. Now, what I want to happen is that during PO creation (whether via ME59 or ME21N), a user exit will auto-populate the Validity End field using a pre-defined formula that will add a couple of days (buffer) to the delivery date.

Is there such a user exit that can auto-populate Validity Start and Validity End?

Thank you,

Aivan