code: private void calculateMonth()
    {
      _dayOfMonth = _dayOfYear;
  
      for (_month = 0; _month < 12; _month++) {
        if (_month == 1 && _isLeapYear) {
          if (_dayOfMonth < 29)
            return;
          else
            _dayOfMonth -= 29;
        }
        else if (_dayOfMonth < DAYS_IN_MONTH[(int) _month])
          return;
        else
          _dayOfMonth -= DAYS_IN_MONTH[(int) _month];
      }
    }
docs: Calculates the month based on the day of the year.