How give a 'timed' online quiz?

Hi Could someone steer me in the right direction for this one - I can't seem to see the forest for the trees. I want to be able to show the user a set of _quiz_ questions - and almost simultaneously (that part's not critical bit) start a _timer_ for the quiz session. The critical bit is that if the preset "time allowed" expires -- I need to interrupt the user, capture their answer up to that point in time, and send them to the examiner. Of course if they finish inside the time limit by submitting the quiz form - we would just kill the session. SO - how do I interrupt the user after a set period of time???? Thanks for any/all clues for this one. Tom