Calendar math .... how to count in week days....?

I've run into a bit of a problem, that I know has been dealt with before, but I don't know how to deal with it. Very simple, I have an application that needs to do calendar math and do it specifically on business days. I need to be able to add and subtract a number of days to only Monday though Friday. Seems simple enough, but I can't figure out how to do it. The only think I've come up with so far is to build a function that counts every day and checks to see if it's a weekend or not. This wouldn't be all that hard to do, but I'm wondering if there is a better way to do it? Anyone have a really efficient function to do this, I need to figure out a LOT of dates in this app. - Anthony