Expiration

Hello, i'm trying to get to work Expiration. I creating system which add time to Expiration and stack if someone clicked some offer. For example: User clicked in captive portal offer at 8:00 and it gives a Expiration to 8:10. Then user clicked again offer in captive portal at 8:05 and it gives time to 8:20. But my problem is: if user log in in 8:00 it will disconnect him at 8:10 and he need to "relog" to system. I want to make a "update" that will not disconnect user at 8:10 but at 8:20 , because time will be stacked. Sorry for my english, hope you understand something. Jakub