On somewhat of a sidebar, I did a report once that required a working days calculation. The client was a manufacturer and they had a shop calendar for their MRP, Shop Floor Scheduling, etc. I read that calendar file to get what they considered a working day, and did my calculations from there. It also had the advantage of being user maintained and multi-depatmental -- for example certain departments would go "down" during certain times of the year due to the seasonal nature of the business, annual maintenance, etc.
Food for thought. Software Support for Sage Mas90, Macola, Crystal Reports and Goldmine