Is there a way to format the Date field to store just the month?

We are looking to add a Billing Month field that is just the Month. In the future I would like to add a report that I can retrieve all the Accounts that are to be billed. 

The reason for wanting to use the Date field is we will also have a custom field billing frequency that will have YEARLY, QUARTERLY . I want to be able to do the math against the month to have my QUARTERLY payers come up on my report correctly.

Any suggestions would be appreciated.