public function convert_to_timestamp($year, $month, $day, $hour = 0, $minute = 0) {
        $gregorianinfo = $this->convert_to_gregorian($year, $month, $day, $hour, $minute);
        return make_timestamp(
            $gregorianinfo['year'],
            $gregorianinfo['month'],
            $gregorianinfo['day'],
            $gregorianinfo['hour'],
            $gregorianinfo['minute'],
            0);
    }