public function durationDiff(Period $period, $get_as_seconds = false)
	    {
	        $diff =   $this->end->getTimestamp()
	                - $this->start->getTimestamp()
	                - $period->end->getTimestamp()
	                + $period->start->getTimestamp();
	        
	        if (! $get_as_seconds) {
	            $res = new DateInterval('PT'.abs($diff).'S');
	            
	            if (0 > $diff) {
	                $res->invert = 1;
	            }

	            return $res;
	        }

	        return $diff;
	    }