Insert Excel Formula Function Analysis 2.2

Morning Experts.

I hope someone can resolve a issue i am facing.

Basically i have two dates "Document Date" and "Due Date"

I want to see the different in days between the dates.

I inserted a column using the "New Cells"function and inserted the excel function =DatedIF(Cell,Cell,"D") function which works perfectly fine in excel 2016, but when i use it as part of the BO workbook it comes up with NA

Am i doing something wrong?

Thanks in advance

Rob