private void checkUnits(){
    boolean success = false;
    if(fFrom.unitsAllPresent(Unit.YEAR, Unit.MONTH, Unit.DAY, Unit.HOUR, Unit.MINUTE, Unit.SECOND) ){
      success = true;
    }
    else if( fFrom.unitsAllPresent(Unit.YEAR, Unit.MONTH, Unit.DAY) &&  fFrom.unitsAllAbsent(Unit.HOUR, Unit.MINUTE, Unit.SECOND) ){
      success = true;
    }
    else if ( fFrom.unitsAllAbsent(Unit.YEAR, Unit.MONTH, Unit.DAY) && fFrom.unitsAllPresent(Unit.HOUR, Unit.MINUTE, Unit.SECOND) ){
      success = true;
    }
    else {
      success = false;
    }
    if(! success ){
      throw new IllegalArgumentException("For interval calculations, DateTime must have year-month-day, or hour-minute-second, or both.");
    }
  }