In excel, I have a simple DATEDIF function and it provides both positive and negative numbers with no problem
Are you sure about this? It does not in my Excel.
In any case, just subtract the two dates. Dates are stored as numbers with 1 = 1/1/1900, 2 = 1/2/1900, etc.
=(RDC Deadline) - TODAY()
Keep in mind that TODAY() is not recalculated each time the list is viewed. It is only recalculated when the list item is edited. (Create a test list and add a calculated column with =”NOW()”, add a few items to the list, note the times, and then refresh the page and note that the times do not change. Edit one of the items and note that only that one has its time updated.)