Now my problem here is that when i click refresh button the timer resets itself and that is WRONG because that will give the users extra time to finsh the exam paper.
I googled and all they were suggesting me is to handle it using session rather then disabling.
Any suggestions please ?
Come on, it's not that hard. Whatever generates that page will also put the current time into the session. That's the basis for your timer. If you need to notify the user immediately when the timer expires, you can send an AJAX request every second which compares the current time to the time in the session and see if it's expired.