vue前端日期返回值错误

formatDate(dateStr) {
      const date = new Date(dateStr);
      const year = date.getFullYear();
      const month = String(date.getMonth() + 1).padStart(2, '0');
      const day = String(date.getDate()).padStart(2, '0');
      const hour = String(date.getHours()).padStart(2, '0');
      const minute = String(date.getMinutes()).padStart(2, '0');
      const second = String(date.getSeconds()).padStart(2, '0');
      return `${year}/${month}/${day} ${hour}:${minute}:${second}`;
    },
    onSubmit() {
      this.form.birthday=this.formatDate(this.form.birthday)
      addStudent(this.form).then(
        res=>{
          console.log("提交"+res)
        }
      )
    }