How to add a "Last Updated at" field in footer area?
Not all page types have a a $node object associated (the user page, for one does not).  You can get rid of the error message that says:

Notice: Undefined variable: node


by wrapping the call to format_date in an if-statement like this:

&lt;?php
if (isset($node-&gt;changed)) {
  print "Last modified date: " . format_date($node-&gt;changed);
}
?&gt;