以下是一个基于 vue 3 实现的简单日历组件的代码示例。这个日历组件包含了前一个月、当前月、下一个月的日期，并且可以支持选择日期、切换月份等功能。

<template>
  <div class="calendar">
    <div class="header">
      <button class="prev" @click="prevMonth"><</button>
      <div class="title">{{ title }}</div>
      <button class="next" @click="nextMonth">></button>
    </div>
    <div class="weekdays">
      <div v-for="day in daysOfWeek" :key="day" class="day">{{ day }}</div>
    </div>
    <div class="days">
      <div
        v-for="day in days"
        :key="day.date"
        :class="{
          today: isToday(day),
          selected: isSelected(day),
          notCurrentMonth: isNotCurrentMonth(day),
        }"
        @click="select(day)"
      >
        {{ day.day }}
      </div>
    </div>
  </div>
</template>

<script>
  import { ref, computed } from "vue";

  export default {
    name: "FeiCalendar",
    props: {
      selectedDate: Date,
    },
    emits: ["update:selectedDate"],
    setup(props, { emit }) {
      const weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"];
      const currentDate = ref(new Date());
      const selectedDate = ref(props.selectedDate || currentDate.value);

      const daysOfWeek = computed(() => {
        return weekdays;
      });

      const days = computed(() => {
        const year = currentDate.value.getFullYear();
        const month = currentDate.value.getMonth();
        const daysInMonth = new Date(year, month + 1, 0).getDate();
        const daysInLastMonth = new Date(year, month, 0).getDate();
        const firstDayOfMonth = new Date(year, month, 1).getDay();

        const days = [];
        let day = 1;
        let lastMonthDay = daysInLastMonth - firstDayOfMonth + 1;
        let nextMonthDay = 1;

        for (let i = 0; i < 6 * 7; i++) {
          if (i < firstDayOfMonth) {
            days.push({
              date: new Date(year, month - 1, lastMonthDay),
              day: lastMonthDay,
              isLastMonth: true,
              isNextMonth: false,
            });
            lastMonthDay++;
          } else if (i >= firstDayOfMonth + daysInMonth) {
            days.push({
              date: new Date(year, month + 1, nextMonthDay),
              day: nextMonthDay,
              isLastMonth: false,
              isNextMonth: true,
            });
            nextMonthDay++;
          } else {
            const date = new Date(year, month, day);
            days.push({ date, day, isLastMonth: false, isNextMonth: false });
            day++;
          }
        }

        return days;
      });

      const title = computed(
        () =>
          `${currentDate.value.toLocaleString("default", {
            month: "long",
          })} ${currentDate.value.getFullYear()}`
      );

      const prevMonth = () => {
        currentDate.value = new Date(
          currentDate.value.getFullYear(),
          currentDate.value.getMonth() - 1,
          1
        );
      };

      const nextMonth = () => {
        currentDate.value = new Date(
          currentDate.value.getFullYear(),
          currentDate.value.getMonth() + 1,
          1
        );
      };

      const isToday = (day) => {
        const today = new Date();
        return day.date.toDateString() === today.toDateString();
      };

      const isSelected = (day) => {
        return day.date.toDateString() === selectedDate.value.toDateString();
      };

      const isNotCurrentMonth = (day) => {
        return day.isLastMonth || day.isNextMonth;
      };

      const select = (day) => {
        selectedDate.value = day.date;
        emit("update:selectedDate", day.date);
      };

      return {
        daysOfWeek,
        days,
        title,
        prevMonth,
        nextMonth,
        isToday,
        isSelected,
        isNotCurrentMonth,
        select,
      };
    },
  };
</script>

<style>
  .calendar {
    max-width: 500px;
    margin: 0 auto;
    font-family: Arial, sans-serif;
  }

  .header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 10px;
  }

  .title {
    font-size: 18px;
    font-weight: bold;
  }

  .weekdays {
    display: flex;
    justify-content: space-around;
    margin-bottom: 10px;
  }

  .day {
    width: 30px;
    height: 30px;
    display: flex;
    justify-content: center;
    align-items: center;
    border-radius: 50%;
  }

  .days {
    display: grid;
    grid-template-columns: repeat(7, 1fr);
    grid-gap: 10px;
  }

  .today {
    background-color: lightblue;
  }

  .selected {
    background-color: blue;
    color: white;
  }

  .notCurrentMonth {
    color: #ccc;
  }
</style>

使用该组件时，可以将selectedDate属性绑定到一个父组件中的数据，这个数据将会存储选中的日期。例如：

<template>
  <div>
    <!-- 用法一 -->
    <FeiCalendar
      :selectedDate="selectedDate"
      @update:selectedDate="onSelectedDateUpdated"
    />
    <!-- 用法二 -->
    <!-- <FeiCalendar v-model:selectedDate="selectedDate" /> -->
    <p>Selected date: {{ selectedDate }}</p>
  </div>
</template>

<script>
  import FeiCalendar from "./FeiCalendar.vue";

  export default {
    components: {
      FeiCalendar,
    },
    data() {
      return {
        selectedDate: new Date(),
      };
    },
    watch: {
      selectedDate(nv) {
        console.log("nv", nv);
      },
    },
    methods: {
      onSelectedDateUpdated(selectedDate) {
        this.selectedDate = selectedDate;
      },
    },
  };
</script>

这是一个简单的示例，可以根据自己的需求对代码进行修改和扩展。