Why is Last-Modified not installed in JOOMLA?

Question

Joomla

The title itself is not set. Tried to display before echo $app, at the very top of the template. Displayed with the header function, output like this:

jimport('joomla.environment.response');
JResponse::allowCache(true);
$dt = gmdate('D, d M Y H:i:s', time()).' GMT';
JResponse::setHeader('Date', $dt, true );
JResponse::setHeader('Last-Modified', $dt, true );
JResponse::setHeader('Cache-Control', 'max-age=300,must-revalidate', true );

Cache-Control is output, Last-Modified is not. Who will tell?