logical test If() not working; data type for the return problem?

I'm doing a logical test off a string type column for inserting a column.  If it is true, I want it to return the number of days as shown below.  If not, i want the string "HBP to be returned.

Is there a convention I have to use to make this work?  I tried the If() simply using strings with no errors.  The same with the DateDiff().  Combining them is the problem, which leads me to believe it's a data type error.  Below is the code.

If([hbp]="N", DateDiff([exp],DateTimeNow()), "HBP")


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