Don't know whether this module may help, or even give you ideas. You will need to log in to see/download:
http://www.redevolution.com/option,com_docman/task,cat_view/gid,14/Itemid,41/
, it says that it is a simple module thatlooks for the most recently created or edited item in the content table and uses it for the latest date to output in a 'Site last updated' module.