public static function nextDate(/* string */$today) {
        list($yyyy, $mm, $dd) = explode('-', $today);
        $isLeap = ($yyyy%400==0) || ($yyyy%4==0 && $yyyy %100 != 0);
        $yearMonth = [0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
        if ($isLeap) {
            $yearMonth[2] += 1;
        }
        // next day
        $dd += 1;
        if ($dd > $yearMonth[$mm]) {
            $mm += 1;
            $dd = 1;
        }
        if ($mm > 12) {
            $yyyy += 1;
            $mm = 1;
        }
        return sprintf("%04d-%02d-%02d", $yyyy, $mm, $dd);
    }