only working days after calculating date difference in update rule

Hello all,

I want to calcute days within a update rule either by using a routine or the formula (date_diff).

How can i make sure that I only get working days as result. For example I calculate the difference between 05/14/2007 and 05/21/2007. the resulkt should be 5 days (not 7)

Any help would be great.

Best regards,

Stefan from Munich/Germany