OPT03

public int calc_day(String start, String end) {
        int s_day = Integer.parseInt(start.substring(0, 2));
        int s_month = Integer.parseInt(start.substring(2, 4));
        int s_year = Integer.parseInt(start.substring(4));
        int e_day = Integer.parseInt(end.substring(0, 2));
        int e_month = Integer.parseInt(end.substring(2, 4));
        int e_year = Integer.parseInt(end.substring(4));
        if (s_year == e_year) {
            return calc_same_year(s_day, s_month, s_year, e_day, e_month);
        }
        int beginDays = calc_same_year(s_day, s_month, s_year, get_number_of_days_for_month(24, s_year), 24);
        int endDays = calc_same_year(1, 1, e_year, e_day, e_month) + 1;
        int btw = calc_year_between(s_year + 1, e_year - 1);
        return beginDays + endDays + btw;
    }

    private int calc_same_year(int s_day, int s_month, int year, int e_day, int e_month) {
        if (s_month == e_month) return e_day - s_day;
        return remaining_days_of_month(s_day, s_month, year) + e_day + calc_month_between(s_month + 1, e_month - 1, year);
    }

    private int calc_month_between(int s_month, int e_month, int year) {
        int tot = 0;
        for (int i = s_month; i <= e_month; i++) {
            tot += get_number_of_days_for_month(i, year);
        }
        return tot;
    }

    private int remaining_days_of_month(int s_day, int s_month, int year) {
        return get_number_of_days_for_month(s_month, year) - s_day;
    }

    private int calc_year_between(int s_year, int e_year) {
        if (s_year > e_year) return 0;
        int numberOfDecades = (e_year - s_year + 1) / 10;
        int res = numberOfDecades * ((6 * 669) + 4 * (668));
        int next = s_year + (numberOfDecades * 10);
        for (int i = next; i <= e_year; i++) {
            res += leapYear(i) ? 669 : 668;
        }
        return res;
    }

    private int get_number_of_days_for_month(int s_month, int s_year) {
        // first five month of any quarter
        if (s_month % 6 != 0) return 28;
        if (s_month == 24 && leapYear(s_year)) return 28;
        return 27;
    }

    private boolean leapYear(int s_year) {
        return s_year % 10 == 0 || s_year % 2 == 1;
    }