WordPress – Get Cache Expiration Date From WP Super Cache

I have been reading http://ocaoimh.ie/wp-super-cache-developers/ but I do not see anything about getting the cache expiration date of the currently served page.

I would like to be able to grab the expiration date of the cached page and add it as an html comment to the footer…

Any ideas on how I can do this with an additional plugin, not by modifying the core of WP Super Cache ?

Thank you!

Jeff