I have identified another issue with the date difference function for weekdays, this time in version 1.16. If a date range ends on January 3rd, 2011, it will calculate an extra day.
For instance, the date difference for week days between Friday, December 31 and Monday, January 3rd is two, when it should be one.
If the range is extended to January 4th, the difference remains at two. I used the test form I created for the original issue to test this issue.
(note, I corrected the date, which is 2011)