Years ago I had written a trigger script for MKS/PTC that did that and posted it in the "Fields Developped Objects" database. Ask PTC support to send you "calculateField.js".
What this means, of course, is that you must use a trigger and cannot use a field computation. Are you sure it's the weekdaysDiff function that fails and not somewhere else in your formula? You may want to post it and have other eyes look at it.