_focusOtherDay(days) {
    const focusedDate = getDateForDateKey(this.state.focusedDateKey);
    const currentMonth = focusedDate.getMonth();

    const nextFocusedDate = getDateForDateKey(this.state.focusedDateKey);
    nextFocusedDate.setDate(nextFocusedDate.getDate() + days);
    const nextFocusedDateKey = convertDateToDateKey(nextFocusedDate);
    const nextMonth = nextFocusedDate.getMonth();

    if (nextMonth !== currentMonth) {
      if ((nextMonth < currentMonth || (nextMonth === 11 && currentMonth === 0)) &&
          !(nextMonth === 0 && currentMonth === 11)) {
        this._decreaseMonthYear();
      } else if ((nextMonth > currentMonth || (nextMonth === 0 && currentMonth === 11)) &&
          !(nextMonth === 11 && currentMonth === 0)) {
        this._increaseMonthYear();
      }
    }

    this.setState({
      focusedDateKey: nextFocusedDateKey,
    });
  }
The function is mainly used when some day is focused and Arrow keys are pressed to navigate to some other day.
days is the number of days by which focused should be moved ahead or behind.