Crystal Reports reading records backwards

Message

     Let me start by stating that I am a novice , at best, when it comes to Crystal Reports. I am using Ver 11.5 in a MRP database.

I have a purchase order file with a current due date field. I need to calculate the po due date by subtracting the stock lead time from the current due date which is simple to do. The problem is that i need to eliminate "non work" days from the equation. I also have a "shop calendar" file which consists of 2 fields; the calendar date and a boolean field that is true if it is a "work" day and false if a "non work" day.  The current due date in the purchase order file is linked to the calendar date in the shop calendar file. 
How do I "read backwards" the shop calendar file until I have the correct po due date ?
Here is an example:
current po due date = 6-19-12
stock lead time = 3 days
Shop calendar records:
6-19-12  true
6-18-12  true
6-17-12  false
6-16-12  false
6-15-12  true
6-14-12  true
The po due date would be 6-14-12

Any help would be appreciated,
John