Count if a date is less than 21 days using IIF and DateDiff

I am working on a report that will display a count when the Create_dt is < 21 days but I'm not having any luck. I have this so far but it is not working and is throwing an error.  Any thoughts would be greatly appreciated.

=SUM(IIf(DateDiff(DateInterval.Day,Fields!Create_dt.Value,Fields!Create_dt.Value + 21) >= 21, 0, 1),)