_decreaseMonthYear() {
    let newMonth;
    let newYear;
    if (this.state.month === 0) {
      newMonth = 11;
      newYear = this.state.year - 1;
    } else {
      newMonth = this.state.month - 1;
      newYear = this.state.year;
    }

    this.setState({
      month: newMonth,
      year: newYear,
      focusedDateKey: undefined,
      lastHoveredDay: undefined,
    });
    if (this.props.onMonthUpdate) {
      this.props.onMonthUpdate(newMonth + 1, newYear);
    }
  }
The function will decrease current month in state. It will also call props.onMonthUpdate.