public function isExpired($sessionTime) {
		
		if (is_null($this->startTime)) {
			return TRUE;
		}

		// creates expiring date subtracting sessionTime interval
		$expiring = new \DateTime(NULL, new \DateTimeZone(BASE_TIMEZONE));
		$expiring->sub(new \DateInterval('PT' . (int)$sessionTime . 'M'));
		
		return ($this->startTime < $expiring);

	}