validation for validity start and validity end dates in me23n

Hi,

A background job is run to create Shipment cost document through VI06 ( a servive PO also gets created ) . We need to

update service po validity start & validity end date (In ME23N : in header data additional data Tab )with the pricing date of cost doc.(PRSDT) .

How can i do this .

Regards ,

shilpa .