Add Hours to Javascript Time

add_hours.js

function addHourTo(start, duration) {
  start = start.split(':');
  duration = duration.split(':');

  totalHours = parseInt(start[0], 10) + parseInt(duration[0], 10);
  totalMinutes = parseInt(start[1], 10) + parseInt(duration[1], 10);

  if (totalMinutes >= 60) {
      totalMinutes -= 60;
      totalHours += 1;
  }

  result = zeroOnLeft(totalHours) + ":" + zeroOnLeft(totalMinutes);
  return result;
}

function zeroOnLeft( n ){
  return ( n < 10 ? "0" + n : n);
}


//Usage:
var finish_time = addHourTo('15:45','00:30');
//finish_time = '16:15'