Measuring of Original Scheduled and Promise Dates on OM Orders

We have a requirement when rescheduling a Sales Order scheduled date and promise date the need to capture the Original Dates for metrics.  For example order was scheduled to ship on 10-OCT-2013.  Order is past due and Planning has recommitted to a 16-OCT-2013 shipment date.  We want the system to reflect the correct planning date, but also need to capture what was the Original and calculate % of On Time Delivery metrics.  Any suggestions?