duration.js

// Sums duration of videos on youtube playlist pages
var total = 0;
var mult = [1, 60, 3600, 86400];

Array.prototype.map.call(document.getElementsByClassName('timestamp'),
  function(a) {
    var time = a.textContent.split(':').reverse()
    for ( var i in time ) total += Number(time[i]) * mult[i];
  }
);

var seconds = total % 60;
var minutes = ((total - seconds) / 60) % 60;
var hours = (total - (minutes * 60 + seconds )) / 3600;

console.log(`${hours}:${minutes}:${seconds}`);