Select elementui --- date format

In doing data submitted by elementui, the default time format of an object, good trouble, mainly on the time limit format, as follows:

<el-form-item :label="$t('oneCard.birthday')+':'" name="出生日期" prop="birthday" style="width:95%;">
  <el-date-picker v-model="temp.birthday" type="date" value-format='yyyy-MM-dd' style="width:120px;" :placeholder="$t('oneCard.birthday')" />
</el-form-item>