Session remaining time count down indicator

https://docs.google.com/file/d/0B0qXqpUMxJDKNFRBUTNYNU9zVVE/edit?usp=sharing
I have a  Session remaining time count down  indicator on my menu, I intend to use it to tell user how many remaining time they have in the current session for security requirement.
It's not down yet.
I am using javascript to show the indicator. I am looking for a way to:
1. After time is up, promp a window and ask user if s/he needs to extend the session to another one hour
2. if no response from user, the current session is over and the current window will be redirected to a designated page.
How can I achieve this?
Thank you very much.
BTW: I tried ...