If you're running Joomla 3.4.4, updating to Mosets Tree 3.6.6 will fix this error.
If you do not want to run the full update, here's the modification you can do to fix this error.
FILE: /administrator/components/com_mtree/mtree.php LINE 779:
PHP Code:
$form->setValue('publish_down', 'publishing', JText::_( 'COM_MTREE_NEVER' ));
Replace with the following:
PHP Code:
$form->setValue('publish_down', 'publishing', $database->getNullDate());