Kristyna Hughes calculates weekdays on the fly:
Knowing the days between events is a fairly common reporting request because a lot of reporting is created to track SLA’s (service level agreement) and other KPI’s (key performance indicators). While getting the days between two dates is fairly easy to achieve, they tend to follow up and ask how many week days there are between two timed events. For example, one company may have a SLA to ship an order within three week days of the order being placed or else a discount is applied to the order. In this case, I would highly recommend that the company have software that calculates these days in the background and stores the actual week days between order date and ship date in a database. Unfortunately, many companies create policies like this without considering future reporting needs and these values have to be calculated on the backend.
Click through for the code but be sure to read the note that this is all weekdays, including holidays.