public function isValid()
    {
        // check that attribute aren't null
        if (
            $this->getFromDay() == null || $this->getFromHour() == null ||
            $this->getToDay() == null || $this->getToHour() == null
        ) {
            return false;
        }

        // check ranges
        if ($this->getFromDay()  < 1 || $this->getFromDay() > 31) {
            return false;
        }
        if ($this->getToDay() < 1 || $this->getToDay() > 31) {
            return false;
        }
        if ($this->getFromHour() > 24 || $this->getToHour() > 24) {
            return false;
        }

        if ($this->getFromDay() == $this->getToDay() && $this->getFromHour() >= $this->getToHour()) {
            return false;
        }

        return true;
    }