Subtract Two Dates To Get Number Of Week Days - Networkdays Does Not Work

I am trying to find a formula that will return the number of week days between two dates. My specific situation is that my job sets up work orders (WO) to be completed by our staff. We have 3 dates - the date the WO was created, the date the WO is due to be completed, and the date the WO was actually completed.

I would like to subtract the Complete date from the Due date. Generally, this should always equal zero because our staff should be completing WOs on the due date! But obviously that doesn't always happen. There are times that they complete them late, and times they complete them early (yay!).

The problem with NETWORKDAYS is that even when they are completed on time, the result is 1. This formula counts instead of subtracts. I adjusted the formula to =NETWORKDAYS(A3,A4)-1 which works fine for those WOs completed on time or completed late. But for those completed early, it adds (or subtracts, really) 2 days. So for a WO completed a day early, instead of it showing -1, it shows -3. I've attached an example of WOs and the NETWORKDAYS formula I've used so you can see.

Subtract Days.xls

I'm really looking for something that will subtract week days, not count them.