Dino, 

(edit, I just finished and looked closer and did not take into account business days mentioned in your sample expression)

I created something that works based on expressions. I can make the date values change, but I don't use them to calculate anything beyond that. 

I added a drop down and 2 data labels. 

DataLabel1: I called dueDate

Datalabel2: I called lexp] and I made it hidden 

I added all the expressions to nexp]

I created expressions low, medium, and high in the format

Add Days( Today(), x)

Where X is 2, 10, or 30

I selected (none) for the oexp] datalabel, it was just there to create the expressions

On my rules, I have, "When DropDown is changed" 

if DropDown does not contain a value

-Transfer data (Check dueDate to blank out the value)

if DropDown contain high

-Transfer Data (in the context browser expand Expressions, move e(x) high into control dueDate)

Repeat for medium and low

Change the Data Label dueDate Data Type to Date

Change the styles to the date format the you are looking for.  

I have the date dispaying, and changes when I change the drop down, but I don't have the date doing anything such as something fires on the date 2 days from now. 

Chad