Funny you should mention that - @Melissa were talking about that yesterday in another context. There are lots of ways to do this, but I think the easiest way is to include the NETWORKDAYS custom function within your date table. See this forum post for details:
Then what you will do, is subtract the NETWORKDAYS column values associated with your beginning and end date values to give you the duration not including weekend days (you can also exclude holidays if you want).
Take a look at this, and if you have any questions about implementing this within your calculations, I’m happy to work through the details with you.
- Brian