Alert user before session expires

Alert user before session expires

In my application session expire time is set as 30 mins. Application will
alert user "your session is about to expire" if the user is inactive for
some times.
I have modified the code to display the alert "your session is about to
expire" at 25th min. Alert will have two buttons "Ok" to continue wit the
session and "Cancel" to logout. If suppose the user noticed the alert
after 30 mins, even if they click on "Ok" it should not allow them. It
should display "your session expired" alert. How to handle this? Also If
the user noticed the alert before session expires(before 30 mins), need
the user to continue wit same session. How to do?
Thanks