public function checkdate($defaultonow = true) {
        if (!checkdate($this->month, $this->day, $this->year)) {
            if ($defaultonow) {
                $now = usergetdate(time());
                $this->day = intval($now['mday']);
                $this->month = intval($now['mon']);
                $this->year = intval($now['year']);
                return true;
            } else {
                throw new moodle_exception('invaliddate');
            }
        }
        return true;
    }