SOURCE

 seconds_end="5200"
let hour_end = Math.floor(seconds_end / 3600) >= 10 ? Math.floor(seconds_end / 3600) : '0' + Math.floor(seconds_end / 3600);
seconds_end -= 3600 * hour_end;
let min_end = Math.floor(seconds_end / 60) >= 10 ? Math.floor(seconds_end / 60) : '0' + Math.floor(seconds_end / 60);
seconds_end -= 60 * min_end;
let sec_end = seconds_end >= 10 ? seconds_end : '0' + seconds_end;
console.log(hour_end + ':' + min_end + ':' + sec_end)