I modified the core's Bartik theme file, node.tpl.php. I simply added
echo "Last updated by $name on " . format_date($node->changed);
The problem is that when there is a new core update, my change will be overwritten. Is there a way to create a module that will easily add this simple line in the node.tpl.php of Bartik's node.tpl.php theme file so that future updates will not overwrite my change?