formula for commit date minus completion date excluding weekends

Hello, I'm trying to calculate the number of week days a project is either delayed or ahead of schedule
Here is a example:
Commit date is Friday 12/21/2012 and the actual completion date is Monday 12/24/2012. The net delay should be +1
Commit date is Monday 12/24/2012 and the actual completion date is Friday 12/21/2012. The net delay should be -1

The formula I'm using is the following:
=NETWORKDAYS(commit date,completion date)

But when i use this function the returned number is either +2 or -2 in stead of +1 or -1

What am i doing wrong, can anyone help me fix this?
I'm using Excel 2007

Joe