JSFiddle - React, Tailwind, and code Playground

by Göran Andersson

JavaScript

var intime = '00:30:05';
var out = '01:07:40';

function toSeconds(s) {
  var p = s.split(':');
  return parseInt(p[0], 10) * 3600 + parseInt(p[1], 10) * 60 + parseInt(p[2], 10);
}

function fill(s, digits) {
  s = s.toString();
  while (s.length < digits) s = '0' + s;
  return s;
}

var sec = toSeconds(intime) + toSeconds(out);

var result =
  fill(Math.floor(sec / 3600), 2) + ':' +
  fill(Math.floor(sec / 60) % 60, 2) + ':' +
  fill(sec % 60, 2);

alert(result);