Date Calculation Fails in Chart Expression to Filter Dimension

[FerdiSisko]

I have a strange issue. When I use a text chart object, I am able to calculate the difference between two dates:

Max(WarrantyDate) - Min(WarrantyDate): 11/10/2020 - 11/08/2020  = 2

However, when I try the same thing for a chart table dimension filter, it doesn't work:

If(Num(Max(WarrantyDate) - Min(WarrantyDate)) = [Number Warranty Days],  [Number Warranty Days])

The subtraction to determine the date difference doesn't work.  I have tried using a variable to store the difference and reference it but while it works for displaying the result in a text object, it also is useless in a chart expression.