Date Difference as Characteristic

I would like to create a <u>characteristic</u> field for the difference between two dates in a query. I have 2 dates, Goods Issue Date, and Shelf Life Expiration Date and I would like to create a characteristic (not a key figure) that has the difference between those two dates. I have seen several examples of how to create a key figure but I cannot find examples to create a characteristic.

Thanks!