When using the DateDiff function with a datepart of "w" (Weekdays), you get the number of weeks between the two dates instead of Weekdays between the two dates. This is exactly the same as using a date part of "ww" (Weeks). The docs are weird because they say "w: Weekdays (same as ww)". My guess is this was originally a bug and then the docs were just updated to reflect the bug instead of just fixing it or something.
Steps to Reproduce:
weeks = dateDiff("w","2016-06-11","2016-06-21");
Build your own Weekdays function