site last updated

hi i am looking for a simple way to find (and then display) the date of the last change to an entire web site (not the date of a single page, like with getlastmod()). exactly like the footer on php.net: "Last updated: Sat Nov 3 13:10:40 2001 EST" in the php.general newsgroup, i couldn't get an answer. will you let me know how you did it for php.net? is a shell script necessary (linux knows the date: the modification date of the site's root directory)? thanks for your help oliver walkhoff