In 5.3 the long time format isn't supported in main.inc.php but is an available option in theme.ini.php.
You can add support by adding this to main.inc.php at or about line 634. Look for the function [:blue]convert time and add this before or after the block that sets the format. I also converted that same block to elseif's to help speed it up a pinch.
if ($theme['timeformat'] == "long") {
$time = date("D M d h:i A", $time);
}