When you serve up the first page, set the javascript timer in the html, and a session timer in global.asa.
The javascript timer can send them to the "times up" page when it expires.
The code that does the corrections, can check the session timer.
Intermediate pages can also check the session timer and update the javascript timer if the two are out of synch (timers using different technologies on different platforms are +- to some degree).
Depending upon the length of the test, you may need to change the session timeout variable in global.asa as most hosts use the default of 20 minutes.