/**
* +/-1 day
* +/-1 week
* +/-2 week
* +/-1 month
* +/-30 days
* +/-1 week 2 days 4 hours 2 seconds
* @param string $interval
* @return JO_Date
*/
public static function setInterval($interval)
{
self::$date = strtotime(self::$date . ' ' . $interval);
return self::$instance;
}
