Add two string time values (HH:mm:ss) with javascript

add_two_times.js

/**
 * Add two string time values (HH:mm:ss) with javascript
 *
 * Usage:
 *  > addTimes('04:20:10', '21:15:10');
 *  > "25:35:20"
 *  > addTimes('04:35:10', '21:35:10');
 *  > "26:10:20"
 *  > addTimes('30:59', '17:10');
 *  > "48:09:00"
 *  > addTimes('19:30:00', '00:30:00');
 *  > "20:00:00"
 *
 * @param {String} startTime  String time format
 * @param {String} endTime  String time format
 * @returns {String}
 */
function addTimes (startTime, endTime) {
  var times = [ 0, 0, 0 ]
  var max = times.length

  var a = (startTime || '').split(':')
  var b = (endTime || '').split(':')

  // normalize time values
  for (var i = 0; i < max; i++) {
    a[i] = isNaN(parseInt(a[i])) ? 0 : parseInt(a[i])
    b[i] = isNaN(parseInt(b[i])) ? 0 : parseInt(b[i])
  }

  // store time values
  for (var i = 0; i < max; i++) {
    times[i] = a[i] + b[i]
  }

  var hours = times[0]
  var minutes = times[1]
  var seconds = times[2]

  if (seconds >= 60) {
    var m = (seconds / 60) << 0
    minutes += m
    seconds -= 60 * m
  }

  if (minutes >= 60) {
    var h = (minutes / 60) << 0
    hours += h
    minutes -= 60 * h
  }

  return ('0' + hours).slice(-2) + ':' + ('0' + minutes).slice(-2) + ':' + ('0' + seconds).slice(-2)
}

I really liked your solution, and I appreciate it so much, it saved me lots of time.
If you may allow me to inform you a bug I found.

My scenario was that I have two inputs one for the checkIn time and another one for checkOut time,
users select the checkIn time so that the checkOut time - which will be 45mins after the checkIn time- can be displayed at the other input checkout input.
I found a bug when the users chose a AM time value, as your function returns an invalid value which won't be displayed at the checkout

After investigation I found that:
when I pass the startTime which is 07:00 for example and the endTime 00:45 your function returns 7:45 which is invalid for inputs 07:45 is the valid one

so I've modified the returned value in my code to look something like this
const formattedHours = Number("0" + hours).toString().length === 2 ? Number("0" + hours) :0${Number("0" + hours)}return formattedHours + ":" + ("0" + minutes).slice(-2);

And it solved my problem, and the checkout value displayed properly at the checkOut input.