Resetting Accounting Records in MySQL

Hi All I work for an isp, and we need to track the users data transfer by calendar month. I have FreeRadius/MySQL running on FreeBSD, the final task for me to do is work out how to force radius to start a new accounting record in the MySQL database which will allow us to do a query by month on the database to calculate data transfer. Is there an obvious solution which I am missing? Thanks in advance Dan