Function transtime

src/date.cpp:878–958

** Given a year, a rule, and the offset from UT at the time that rule takes ** effect, calculate the year-relative time that rule takes effect. */

    ** effect, calculate the year-relative time that rule takes effect.
    */
    static int_fast32_t transtime(const int year, const struct rule *const rulep, const int_fast32_t offset) {
        int	leapyear;
        int_fast32_t value;
        int	d, m1, yy0, yy1, yy2, dow;

        INITIALIZE(value);
        leapyear = isleap(year);
        switch (rulep->r_type) {

        case JULIAN_DAY:
            /*
            ** Jn - Julian day, 1 == January 1, 60 == March 1 even in leap
            ** years.
            ** In non-leap years, or if the day number is 59 or less, just
            ** add SECSPERDAY times the day number-1 to the time of
            ** January 1, midnight, to get the day.
            */
            value = (rulep->r_day - 1) * SECSPERDAY;
            if (leapyear && rulep->r_day >= 60)
                value += SECSPERDAY;
            break;

        case DAY_OF_YEAR:
            /*
            ** n - day of year.
            ** Just add SECSPERDAY times the day number to the time of
            ** January 1, midnight, to get the day.
            */
            value = rulep->r_day * SECSPERDAY;
            break;

        case MONTH_NTH_DAY_OF_WEEK:
            /*
            ** Mm.n.d - nth "dth day" of month m.
            */

            /*
            ** Use Zeller's Congruence to get day-of-week of first day of
            ** month.
            */
            m1 = (rulep->r_mon + 9) % 12 + 1;
            yy0 = (rulep->r_mon <= 2) ? (year - 1) : year;
            yy1 = yy0 / 100;
            yy2 = yy0 % 100;
            dow = ((26 * m1 - 2) / 10 +
                   1 + yy2 + yy2 / 4 + yy1 / 4 - 2 * yy1) % 7;
            if (dow < 0)
                dow += DAYSPERWEEK;

            /*
            ** "dow" is the day-of-week of the first day of the month. Get
            ** the day-of-month (zero-origin) of the first "dow" day of the
            ** month.
            */
            d = rulep->r_day - dow;
            if (d < 0)
                d += DAYSPERWEEK;
            for (int i = 1; i < rulep->r_week; ++i) {