The datediff function is based on time portion of datetime fields (and returns # of seconds).
The weekdaysDiff function returns # of weekdays.
This is a very straightforward request: we need a function that ignores the time portion of datetime fields and returns # DAYS between 2 dates (i.e. as weekdaysDiff does, and, as does Excel's DAYS(field1, field2).
If now() is used as one of the datetime fields in datediff function, from midnight to midnight of a given day, running a report or chart with the datediff function will return different values (i.e. as the seconds increase between now() and the other datetime field, eventually the calculated # of days will increase by 1).