Calculate the number of days between Today and Date Modified

Did you try my formula as a Formula numeric field type?

If you want a numeric field for the # of days , it would be
ToDays(Today() - ToDate([Date Modified]))

Here is what I get:

The expression Today() on the left hand side of the operator "-" is of type datewhile the expression [Date Modified] on the right hand side is of type date/time. The operator "-" cannot be used with these types of expressions.

This is because the [Date Modified] is a Date/Time field, and Today() is a "Date" value. 

Mark's comment about displaying the number of days as a numeric value would be the formula to use:

ToDays(Today() - ToDate([Date Modified]))

Worked like a charm!!!  Thank you kindly.