private void stepDay() {
    if(fIsPlus){
      fDay = fDay + 1;
    }
    else {
      fDay = fDay - 1;
    }
    if(fDay > numDaysInMonth()){
      fDay = 1;
      stepMonth();
    }
    else if (fDay < 1){
      fDay = numDaysInPreviousMonth();
      stepMonth();
    }
  }