Calculate the difference between two dates

I have a formula numeric field (called Duration) where I'd like to show in days (excluding weekends) the difference between the start date (date field) and end date (date field).  I'm using the following formula, but the number in the Duration field is not right (([End Date:] - [Start Date:]) / Days(30)).  

Please let me know what am I doing wrong.

Thank you!

-g

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Gwen Peters
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