code: function numToDate(num) {
    const str = `${num}`;
    const YN = str.substring(0, 4);
    const M = str.substring(4, 6);
    const D = str.substring(6, 8);
    return `${YN}-${M}-${D}`;
  }
docs: 将 20170102 => 2017-01-02
  @param { number | string } num