Free of charge traffic at night

Hello.

Our 3G-operator provide free traffic at night. So, I need to bill my users by schedule.

I know how to do it by custom script if the whole session falls on the day or on the night. For example:

from 23:00:00 to 00:00:00 - paid session
from 00:00:00 to 01:00:00 - free session

But if sessions crosses midnight (i.e. from 23:00 to 01:00), I can not know how much traffic is paid and how much is free. So, I need the way to split session at midnight.

I use Hotspot and this is a not the best solution just to logout from Hotspot session because the user can downloading something by schedule and not to surf the Web to restart session by cookie.

Is there such command or API call to do it?