I am trying to get the difference between two cells (both from formulas) but get the "Invalid Operation" instead. I know this is simple and I'm overlooking something!
First, pretty cool to see Decatur and Huntsville in your list as I live in Decatur.
Onto your question, can you paste the formula you're using? I compare fields with formulas in them a lot and know that you can do it. I'm wondering if there is something being passed to a function that is returning your error.