SLA according to calendar with holidays and vacations in crm 2015 online

Hi,

We work in crm 2015 online.

I have a dateTime field that called "dateToClose",

And when I save the record I put a value in this field:

var expiryDate = new Date(); //its like DateTime.Now
expiryDate.setDate(expiryDate.getDate() + 12);
Xrm.Page.getAttribute("dateToClose").setValue(expiryDate);

The SLA time Field Failure is dateToClose.
It works very nice.

Now instead of adding 12 days I need to add only 12 days of work.

(withOut holidays and vacations)
How can I do it?

thanks!