Purchase Request

For Delivery date < System date, you can set the message ME 039
Delivery date is in the past to error message.

Request date is the date of Purchase requisition created. It can be changed before you enter the item information. By default system proposes system date. For Request date, you either have to set the field to display mode through field selection control for Purchase requisition or have written abap code in User exit or Badi to control the date.