How to execute availability check against delivery date in purchase order

Hi all

In MD04, I know that "available date = delivery date in purchase order + GR processing time" & "GR date = delivery date in purchase order". When I execute availability check in production order, I will get material's committed date = available date. But I hope to get material's committed date = GR date in purchase order.

For example. delivery date in purchase order is 2012/02/06 and GR processing time is 2 days. After ATP in production order, I will get committed date 2012/02/08. But I hope it show 2012/02/06. How to customize it.

(The GR processing time must be set 2 days for MRP calculation. it can be changed to 0.) Thanks!!