Have a look at this old thread of Joe Mako's for a couple of ideas (whatever happened to Joe - used to be a real regular?).  Best way may be use James's solution for regular week days and a supplementary table like I suggested for public holidays.

http://www.tableausoftware.com/forum/would-it-be-possible-get-count-weekdays-between-two-dates

I'm sure it's doable in Tableau using an approach based on the thread I sent you - but definitely a bit of work and not trivial.  I might try to put some more hints together when I get a minute.

But before I do - just one other thought.  What is your real data source?  Is it Excel, or was that just some sample data?  The reason I ask is that if your data really is coming from Excel, you might find it easier to do this by using Excel formulas to create an extra column in your source data.

Just wanted to check.  How'd yall do on this effort?  I'm embarking on the same effort.

Hi Richard,

The link goes to the main forums page. I tried to search for it but couldn't find it. Anyway we can get access to it now?

Regards,

Jai

EDIT:Found the new link

http://community.tableau.com/thread/104481

Here's a calc for networkdays only, not taking into account any holidays. You could expand it to include business hours.

http://community.tableau.com/message/178728#178728