前端yyyy-mm-dd格式 计算一段工作日后,日期

//计算一段工作日后,日期
getWorkday(dat, itervalByDay) {
  function formatTen(f) {
    if (parseInt(f, 10) < 10) {
      return '0' + f;
    }
    return f;
  }

  let str = dat.split("-");
  let date = new Date();
  date.setUTCFullYear(str[0], str[1] - 1, str[2]);
  date.setUTCHours(0, 0, 0, 0);
  let millisceonds = date.getTime();
  for (let i = 1; i <= itervalByDay; i++) {
    millisceonds += 24 * 60 * 60 * 1000;
    date.setTime(millisceonds);
  }
  let year = date.getFullYear();
  let month = date.getMonth() + 1;
  let day = date.getDate();
  return year + "-" + formatTen(month) + "-" + formatTen(day);
},