As much as Olly succinctly said,
There are a few steps that are required to do that.
First, you need to import a number of user defined functions from the functions master. On a standalone Monarch installation, you'll normally find this model file at C:\Documents and Settings\All Users\Documents\Monarch\Models\UserDefinedFunctions.xmod. Monarch should locate the file automatically when you want to Import user defined functions.
You'll need to import the related WorkingDays functions, and then customize the formulas for ComputeDaysOffInRange, IsWorkingDay, FirstValidDateForWorkingDays, and LastValidDateForWorkingDays.
Finally, you can then use Working_Days() in a new calculated field.
Even with the moderate level of effort required to use Monarch's function, it must be far easier than attempting to do the same accurately in Excel.
That said, I personally haven't had the need to do it yet, but I'm confident that the function has been tested thoroughly and is reliable.