Formula for calculating number of weeks?

Hello, I am working on a spreadsheet that will tell our Inside Sales rep when an order will need to ship from our location in order to make it to Malaysia on time. For example if the shipment needs to be in Malaysia by 12/18/15 I want the formula to back out 10 weeks including weekends and holiday's (this will account for transit and customs). Some of the orders I will need to back out a different number of weeks ie., 6 weeks AND some of them I need to send to a 2nd location (and they need 3 weeks to do what they have to do) for another operation before it ships to the customer.
UTAS-RCC-SCI-Ship-Schedule.xlsx