Calculate Duration (days) between two dates but only calculate weekdays.

Can you answer me this question?  Are counting fences or fence posts?

or put another way

Let's say that weekends aside, how many days are there in say January 2017? You will likely tell me that there are 31 days.

But if I were to take the 31st and subtract the 1st of the month, Let's see that is 31 - 1 = 30 days.  Yikes, someone stole a day from us.

Where did the missing day go?  Well like I said are we counting fences or fence posts?

You will need to "add 1" if you want to count both the first and last day of the period  in the weekdaysub (  )