element ui实现form验证起始时间不能大于结束时间

<el-form-item label="开始时间" :label-width="formLabelWidth" prop="startTime">
　　<el-date-picker v-model="form.startTime" type="datetime" format='yyyy-MM-dd HH:mm' value-format="yyyy-MM-dd HH:mm" style='width: 100%;' :picker-options="pickerOptionsStart" default-time="08:00:00" placeholder="选择开工时间">
　　</el-date-picker>
</el-form-item>
<el-form-item label="结束时间" :label-width="formLabelWidth" prop="endTime">
　　<el-date-picker v-model="form.endTime" type="datetime" format='yyyy-MM-dd HH:mm' value-format="yyyy-MM-dd HH:mm" style='width: 100%;'  :picker-options="pickerOptionsEnd" default-time="20:00:00" placeholder="选择完工时间">
　　</el-date-picker>
</el-form-item>

		data() {
			return {
				form: {
                                        startTime：' ',
                                        endTime: ' '
                                },
				rules: {
					startTime: [{
						required: true,
						message: '请选择开始时间',
						validator: this.pickerOptionsStart,
						trigger: 'change'
					}],
					endTime: [{
						required: true,
						message: '请选择结束时间',
						validator: this.pickerOptionsEnd,
						trigger: 'change'
					}]
				},
				pickerOptionsStart: {
					disabledDate: time => {
						let endDateVal = this.form.endTime;
						if (endDateVal) {
							return time.getTime() > new Date(endDateVal).getTime();
						}
					}
				},
				pickerOptionsEnd: {
					disabledDate: time => {
						let beginDateVal = this.form.startTime;
						if (beginDateVal) {
							return (time.getTime() < new Date(beginDateVal).getTime() - 1 * 24 * 60 * 60 * 1000)
						}
					}
				}
			}
		},