Date substraction on custom report

Date substraction on custom report

I need to substract dates on custom reports. Specifically, I want to be able to calculate how long a negotiation lasted by substracting the creation date of a potential from the closing date of that potential. How can I do it?

Formula custom field doesn't seem to work, it think because creation time is time-date type of field, and due date a date field (type mismatch) but I am not so sure. 
 
Built in criterias for reports don't allow me to compare dates with ranges (only to fixed dates). I could try having multiple reports with ranges of dates, but that seems like a lot of work, I am guessing a better way is available.

Any suggestions?  
Thanks in advance

Maria