vue 判断两个时间插件结束时间必选大于开始时间

<template>
 <div id="date_time_picker">
 
 <van-button plain type="primary" @click="showPopFn()">点击选择日期</van-button>
 <van-field v-model="timeValue" placeholder="选择的日期结果" readonly />
 <van-popup v-model="show" position="bottom" :style="{ height: '40%' }">
  <van-datetime-picker v-model="currentDate" type="date" @change="changeFn()" @confirm="confirmFn()" @cancel="cancelFn()" />
 </van-popup>
 
 </div>
</template>
 
<script>
 export default {
 data() {
  return {
  msg: '',
  currentDate: new Date(),
  changeDate: new Date(),
  show: false, // 用来显示弹出层
  timeValue: ''
  }
 },
 methods: {
  showPopFn() {
  this.show = true;
  },
  showPopup() {
  this.show = true;
  },
  changeFn() { // 值变化是触发
  this.changeDate = this.currentDate // Tue Sep 08 2020 00:00:00 GMT+0800 (中国标准时间)
  },
  confirmFn() { // 确定按钮
  this.timeValue = this.timeFormat(this.currentDate);
  this.show = false;
  },
  cancelFn(){
  this.show = true;
  },
  timeFormat(time) { // 时间格式化 2019-09-08
  let year = time.getFullYear();
  let month = time.getMonth() + 1;
  let day = time.getDate();
  return year + '年' + month + '月' + day + '日'
  }
 },
 mounted() {
  this.timeFormat(new Date());
 }
 }
</script>
 
<style>
</style>

// main.js文件里面的部分代码
import {Button} from 'vant'
import { DatetimePicker } from 'vant';
import { Popup } from 'vant';
import { Field } from 'vant'; 
 
Vue.use(Button)
Vue.use(DatetimePicker)
Vue.use(Popup)
Vue.use(Field);

<van-datetime-picker
 class="datePick"
 v-model="currentDate"
 type="date"
 :min-date="minDate"
 :max-date="maxDate"
  readonly="readonly"
  @confirm="confirmDate"
  @cancel="cancelDate"/>
 
mounted() {
let dateLimit = new Date();
let year = date.getFullYear();
let month = date.getMonth()+1;
let d = new Date(year, month, 0);
//最大允许选择当前日期
this.maxDate = new Date(this.$moment(dateLimit).format('YYYY/MM/DD'));
//最大允许选择当前日期+3个月
this.maxDate = new Date(this.$moment(dateLimit).add('3','M').format('YYYY/MM/DD'));
//最小允许选择当前日期
this.minDate = new Date(this.$moment(dateLimit).format('YYYY/MM/DD'));
//最小允许选择当前日期-3个月
this.minDate = new Date(this.$moment(dateLimit).subtract('3','M').format('YYYY/MM/DD');
//最小允许选择当前日期-3个月（当前月不算，减去三个整月2020/06/01----2020/09/22）
this.minDate = new Date(this.$moment(dateLimit).subtract('3','M').format('YYYY/MM')+'/01');
 
//只允许选择当前月
let year = dateLimit.getFullYear();
let month = dateLimit.getMonth()+1;
let d = new Date(year, month, 0);
let enddate = d.getDate();
this.minDate = new Date(this.$moment(dateLimit).format('YYYY/MM/')+'01');
this.maxDate = new Date(this.$moment(dateLimit).format('YYYY/MM/')+ enddate);
}