code: private long calculateJoin()
    {
      _year += divFloor(_month, 12);
      _month -= 12 * divFloor(_month, 12);
  
      _localTimeOfEpoch
        = MS_PER_DAY * (yearToDayOfEpoch(_year)
                        + monthToDayOfYear(_month, isLeapYear(_year))
                        + _dayOfMonth);
  
      _localTimeOfEpoch += _ms + 1000 * (_second + 60 * (_minute + 60 * _hour));
  
      return _localTimeOfEpoch;
    }
docs: Based on the current data, calculate the time since the epoch.
  
  @return time since the epoch, given the calendar components