Check for Missing Date in Formula Custom Field

I've created a custom field, 'Review Date' that I am trying to get the Formula field to check if anything has been entered and if there is, to calculate based on that field, if not, calculate based on creation date. I looked everywhere for a way to find a 'null' for a date field but couldn't find an answer. No matter what I do, I get "Incorrect data type of argument." How do I check if a date is null or not?

If(
    Contains(${Accounts.Company Status},'Prospecting'),
       If(
        (${Accounts.Review Date}!=null), (Datecomp(Now(),${Accounts.Review Date})/(60*24)),
           If(
            (${Accounts.Review Date}=null), (Datecomp(Now(),${Accounts.Created Time})/(60*24)), 0
              )
          ),
   0)