Quick update - I already have a formula I can use that will do my comparisons partly:

=IF(K4="",(L4-J4),(L4-K4)))
But what I need is that if the result of this formula is blank, then I need to use:

=IF(K5="",J5-(TODAY()),K5-(TODAY()))
Will that work? I've tried fiddling with it and most often just get a "volatile" response!