using the generate attribute operator you can use the Regular expressions, and build a query with the inbuilt date functions. this would work to work the date diff okay, but to leave out the weekends not to sure.
this would be my pseudo code for the reg ex expression.
if date, then days = today - pref date weekday_count = days / 7 * 5 return weekday_count else return date to generate your new attribute "weekday_count" check the "transaction date" attribute for a value. count the difference, to get the total days then work out 5/7 to get the no of week days between the two. return this value otherwise return the date value for the example (i.e unknown)