How do I count the number of days between when an order is opened and when it ships?

How do I count the number of days between when an order is opened and when it ships?

I'm trying to create a formula in CRM. I've tried using a formula like this but I keep getting a syntax error.

((${PurchaseOrders.PO Date} - ${PurchaseOrders.Shipped Date}) / (1000 * 3600 * 24 ))