Determining date difference excluding weekends:

The idea is basically the same in script and formula.  Find the number of full weeks between the date by moving the start date forward to the next sunday, the end date back to the previous saturday.  Multiple by 2/7=number of weekend days.

Then, add 2 (since you definitely excluded the firt saturday/last sunday).  Add another 1 if start=sunday, and another 1 if end=saturday.

startDate := date1;
endDate := date2;
nextSun := @Adjust(startDate; 0; 0; 8-@Weekday(startDate); 0; 0; 0);
prevSat := @Adjust(endDate; 0; 0; -@Weekday(startDate); 0; 0; 0);
easyPart := (@Date(prevSat) - @Date(nextSun)) * 2/7;
easyPart + 2 + (@Weekday(startDate)=1) + (@Weekday(endDate)=7);

Oops, wrong code.

startDate := @Date(start_date_value);
endDate := @Date(end_date_value);
prevSun := @Adjust(startDate; 0; 0; 1-@Weekday(startDate); 0; 0; 0);
nextSat := @Adjust(endDate; 0; 0; 7-@Weekday(endDate); 0; 0; 0);
daysInBetween := (nextSat - prevSun) / 86400+1;
weekEnds :=  daysInBetween * 2/7;
exclusions := @Weekday(startDate)!=1 + @Weekday(endDate)!=7;
weekEnds - exclusions;


Variations on what I described above... adds all weekend days of the weeks involved (inlcudiig 2 weekend days per partial week, i.e., counts 2 days in start week even if partial week).

Then, drops the "first Sunday" if necessary and ditto the "last Saturday" to account for partial weeks (see exclusions variable).

Thnaks to Don at Chase who first came up with this in 1990, I think.