SOURCE

var Main = {
    data() {
      return {
        value: ''
      };
    },
    mounted() {
    	setTimeout(()=> {
      	this.value = '2018-4-25'
      }, 500)
    }
  };
console.log('???', Vue)
var Ctor = Vue.extend(Main)
new Ctor().$mount('#app')

<script src="//unpkg.com/vue/dist/vue.js"></script>
<script src="//unpkg.com/element-ui@2.3.6/lib/index.js"></script>
<div id="app">
<template>
  <div class="block">
    <span class="demonstration">默认</span>
    <el-date-picker v-model="value" type="date" placeholder="选择日期">
    </el-date-picker>
  </div>
</template>
</div>

@import url("//unpkg.com/element-ui@2.3.6/lib/theme-chalk/index.css");