I am using the following formula to calculate workdays
=SUMPRODUCT(INT((MAX(K9,J9)-WEEKDAY(MAX(K9,J9)+1-{2;3;4;5;6})-MIN(K9,J9)+8)/7))

However I am experiencing the following problems:

When no dates are captured in start or end date it calculates an #NUM! error

I have more than one interval where the end date in the one formula becomes
the start date of the next formula. If i capture the end date in the one
calculation but no end date in the next calculation a 1 day total is
calculated while it should be 0 until a end date is completed
--
Andrew