protected function _dateDiff($start = null, $stop = null) {
		if ($start) {
			$endDate = $stop instanceof DateTime ? $stop : new DateTime($stop ? $stop : 'now');
			$startDate = $start instanceof DateTime ? $start : new DateTime($start);
			$diff = $startDate->diff($endDate)->format(Configure::read('Task.dateDiffFormat'));
			return $this->_isCli ? $diff : $this->Html->tag('span', $diff);
		} else {
			return $this->_none();
		}
	}