SOURCE

function transformTime(timestamp = +new Date()) {
    if (timestamp) {
        var time = new Date(timestamp);
        var y = time.getFullYear();
        var M = time.getMonth() + 1;
        var d = time.getDate();
        var h = time.getHours();
        var m = time.getMinutes();
        var s = time.getSeconds();
        return y + '-' + addZero(M) + '-' + addZero(d) + ' ' + addZero(h) + ':' + addZero(m) + ':' + addZero(s);
      } else {
          return '';
      }
}
function addZero(m) {
    return m < 10 ? '0' + m : m;
}
// console.log(transformTime(300006))


/**
 * JS 计算两个时间间隔多久（时分秒）
 * @param startTime "2019-10-23 15:27:23"
 * @param endTime "2019-10-23 15:27:55"
 * @return 1天2时3分5秒
 */
function twoTimeInterval(startTime,endTime) {
 
  // 开始时间
  let d1 = startTime.replace(/\-/g, "/");
  let date1 = new Date(d1);
 
  // 结束时间
  let d2 = endTime.replace(/\-/g, "/");
  let date2 = new Date(d2);
 
  // 时间相差秒数
  let dateDiff = date2.getTime() - date1.getTime();
 
  // 计算出相差天数
  let days = Math.floor(dateDiff / (24 * 3600 * 1000)).toString().padStart(2,'0')
 
  // 计算出小时数
  let residue1 = dateDiff % (24 * 3600 * 1000); // 计算天数后剩余的毫秒数
  let hours = Math.floor(residue1 / (3600 * 1000)).toString().padStart(2,'0')
 
  // 计算相差分钟数
  let residue2 = residue1 % (3600 * 1000); // 计算小时数后剩余的毫秒数
  let minutes = Math.floor(residue2 / (60 * 1000)).toString().padStart(2,'0')
 
  // 计算相差秒数
  let residue3 = residue2 % (60 * 1000); // 计算分钟数后剩余的毫秒数
  let seconds = Math.round(residue3 / 1000).toString().padStart(2,'0')
 
  let returnVal =
    ((hours == 0) ? "" : hours +":") +
    ((minutes == 0) ? "00:" : minutes+":") +
    ((seconds == 0) ? "00" : seconds+"");
 
  return returnVal;
 
}

// console.log(twoTimeInterval('1970-01-01 08:00:00', transformTime(300006)))
console.log(twoTimeInterval('1970-01-01 08:00:00', '1970-01-01 08:12:10'))