Elapsed time billing.

Hi everyone, Thanks to the advice and help I got from the list I now have a fully working FreeRadius server with MySQL. I can issue user names and passwords for set amounts of time e.g. 1 hour or 24 hours.What I'd like to do is issue names and passwords that will last for passed time e.g. one day or one month. Has anyone done this and can you advise me on how to go about it. Thanks for all the help in the past. Regards, Sean