如何计算react-native-calendars中选定的天数？

import React, { useState } from 'react';
import { View } from 'react-native';
import { Calendar } from 'react-native-calendars';

const MyCalendar = () => {
  const [selectedDates, setSelectedDates] = useState({});

  const onDayPress = (day) => {
    const newSelectedDates = { ...selectedDates };
    if (newSelectedDates[day.dateString]) {
      // 若该日期已选中，则取消选中
      delete newSelectedDates[day.dateString];
    } else {
      // 若该日期未选中，则选中
      newSelectedDates[day.dateString] = { selected: true };
    }
    setSelectedDates(newSelectedDates);
    const selectedCount = Object.keys(newSelectedDates).length;
    console.log('选定的天数：', selectedCount);
  };

  return (
    <View>
      <Calendar
        markedDates={selectedDates}
        onDayPress={onDayPress}
        onDayLongPress={onDayPress}
      />
    </View>
  );
};

export default MyCalendar;