Calc duration with JS

<form action="">
  <fieldset>
    <legend>Hora 1</legend>
    <label for="initHour">Hora(s): </label>
    <input type="number" name="initHour" id="initHour" value="0" />

    <label for="initMin">Minuto(s): </label>
    <input type="number" name="initMin" id="initMin" value="0" />

    <label for="initSec">Segundo(s): </label>
    <input type="number" name="initSec" id="initSec" value="0" />
  </fieldset>
  <fieldset>
    <legend>Hora 2</legend>
    <label for="endHour">Hora(s): </label>
    <input type="number" name="endHour" id="endHour" value="0" />

    <label for="endMin">Minuto(s): </label>
    <input type="number" name="endMin" id="endMin" value="0" />

    <label for="endSec">Segundo(s): </label>
    <input type="number" name="endSec" id="endSec" value="0" />
  </fieldset>
  <button type="button" id="sum">Somar</button>
  <button type="button" id="subtract">Subtrair</button>
  <button type="reset">Resetar</button>
</form>
<hr>
<fieldset>
  <legend>Resultado</legend>
  <label for="outHour">Hora(s): </label>
  <input type="text" name="outHour" id="outHour" />

  <label for="outMin">Minuto(s): </label>
  <input type="text" name="outMin" id="outMin" />

  <label for="outSec">Segundo(s): </label>
  <input type="text" name="outSec" id="outSec" />

  <hr>

  <label for="fullOut">Horario Completo: </label>
  <span id="fullOut"></span>
</fieldset>

let initHour = document.querySelector('#initHour');
let initMin = document.querySelector('#initMin');
let initSec = document.querySelector('#initSec');

let endHour = document.querySelector('#endHour');
let endMin = document.querySelector('#endMin');
let endSec = document.querySelector('#endSec');

let outHour = document.querySelector('#outHour');
let outMin = document.querySelector('#outMin');
let outSec = document.querySelector('#outSec');

let fullOut = document.querySelector('#fullOut');

document.querySelector('#sum').addEventListener('click', sum);
document.querySelector('#subtract').addEventListener('click', subtract);

function inverter() {
  let tempHour = initHour.value;
  let tempMin = initMin.value;
  let tempSec = initSec.value;

  initHour.value = endHour.value;
  initMin.value = endMin.value;
  initSec.value = endSec.value;

  endHour.value = tempHour;
  endMin.value = tempMin;
  endSec.value = tempSec;            
}

function subtract() {
  if(Number(initHour.value) < Number(endHour.value)) {
    inverter();
  }
  let time = {};
  let hour = 0;
  let minute = 0;
  let second = 0;

  time = verifyValue(Number(initSec.value), Number(endSec.value));
  second = time.valueOne;
  minute = time.valueTwo;

  time = verifyValue(Number(initMin.value), Number(endMin.value));
  minute += time.valueOne;
  hour = time.valueTwo;

  hour += Math.abs(Number(initHour.value) - Number(endHour.value));
  out(hour, minute, second);
}

function verifyValue(initValue, endValue) {
  let valueOne = 0;
  let valueTwo = 0;
  if(initValue < endValue) {
    valueOne += (initValue+60)- endValue;
    valueTwo--;
  } else {
    valueOne = Math.abs(initValue - endValue);
  }
  return {valueOne, valueTwo};
}

function sum() {
  let hour = Number(initHour.value) + Number(endHour.value);
  let minute = Number(initMin.value) + Number(endMin.value);
  if(minute >= 60) {
    hour++;
    minute -= 60;
  }
  let second = Number(initSec.value) + Number(endSec.value);
  if(second >= 60) {
    minute++;
    second -= 60;
  }
  out(hour, minute, second);
}

function out(hour, minute, second) {
  outHour.value = hour;
  outMin.value = minute;
  outSec.value = second;

  fullOut.innerHTML = `${addZero(hour)}:${addZero(minute)}:${addZero(second)}`;
}

function addZero(input) {
  return input > 9 ? input : `0${input}`;
}