code: function setISOWeekNumber(d, num) {
    if (isNumber(num)) {
      // Intentionally avoiding updateDate here to prevent circular dependencies.
      var isoWeek = cloneDate(d), dow = getWeekday(d);
      moveToFirstDayOfWeekYear(isoWeek, ISO_FIRST_DAY_OF_WEEK, ISO_FIRST_DAY_OF_WEEK_YEAR);
      setDate(isoWeek, getDate(isoWeek) + 7 * (num - 1));
      setYear(d, getYear(isoWeek));
      setMonth(d, getMonth(isoWeek));
      setDate(d, getDate(isoWeek));
      setWeekday(d, dow || 7);
    }
    return d.getTime();
  }
docs: Week number helpers