sqlcounter / end session at the end of the month

Hi, I'm having issues with octet accounting. Users are monthly limited in octets, not in time. The problem i have is when a user logs in on the last day of the month and stays online for 3 days then there's 2days within the new month but the accounting counts within the previous month, which we don't like :-) So I was thinking about adding a counter module which calculates a session-timeout at the last second of the month so users get logged out just before the new month starts. But, I don't know how to set the check-item. If I use max-all-session and a monthly reset I need a check item that equals the end of the month, which can be 28 days, 30 days or 31 days. Any known solutions to this? Kind regards, Y.