I want to display to the user on their Zen page how much time is left before their session times out.

I know I can set the %session.AppTimeout to a particular value (or manage this in the Web Application) and I know I can retrieve this value from the %session object. I also know that I can set an event handler to do something when the session has timed out but I cannot find any way to determine how long before the session ends via the application timeout so that I can keep the user informed.