I'm working in Zoho CRM desperately trying to calculate aging between to dates, but - if the completed date is not inputted (indicating it's still open), I'd like to use now.  I want to return a whole number.  This is what I came up with, but it will not work...

Floor(Datecomp(If(${Implementations.SSL Cert Ticket Completed}==isNull(),Now(),${Implementations.SSL Cert Ticket Completed}),${Implementations.SSL Cert Ticket Date})/1440)

Help - how can I calculate the aging (time in days) it took to go from Implementations.SSL Cert Ticket Date to Implementations.SSL Cert Ticket Completed and use Now if there is not an entry in Implementations.SSL Cert Ticket Completed?