session timeout eval random only during night hours

Hello, basically what i have is: Session-Timeout = 24*60*60 (secons so = one day) what i whould like to eval this timeout every time user gets ack/ok reply to be (randomly) in hours let's say 0200 - 0600 hours. any quick hint? Don't want to reinvent the wheel or use some cannons to shut at butter fly. maybe someone has some advice? thanks.