Hi Ray,
Opening period in days can be defined in schedule margin key OMDC which can be assigned to material master MRP 2 view and in MD02 Create purchase req. =2 , Purchase requisitions in opening period will consider that period.
Please refer the SAP help link,
http://help.sap.com/erp2005_ehp_04/helpdata/EN/f4/7d2dc744af11d182b40000e829fbfe/frameset.htm
Regards,
R.Brahmankar