I usually use this on mine:
< ?php echo "Last modified: ".date( "F d Y H:i:s.", getlastmod() ); ? >
The getlastmod() command looks at the current file. And it also works when the command is actually in a shared file elsewhere. (I usually dump the common header/footer stuff into a seperate PHP file for easy changing. The old empeg logo site this this at a basic level, and RioCar.org does this quite a bit.