Basic Monthly Lead Reporting

Basic Monthly Lead Reporting

Hello, I want to create what seems like a simple report: How many leads did we receive in X month?   This is easy enough if the leads are not converted to potentials. Once a lead moves to a potential file, I lose the lead created date. 

I can easily pull a lead report by date and a potential report by date but the potentials could have been created by a lead we got yesterday or a lead that came in 2 years ago. 

How can I create a report that shows, for example, in "April 2015 we received X leads" (regardless of where they are now, potentials, closed lost, closed won etc...)

Thank you!