I copies and pasted your formula into the super store and it worked fine, no error. Produced this number:
If your calc is a table calc, NOW() and TODAY() used inside the table calc can generate that error. If that's the case, then the workaround is to call NOW() or TODAY() in a separate calc, then Tableau can instantiate that calculated field into the context for use in your other calc.
Thanks to Joe Mako for the tip on that one.