datetime – Countdown click in JavaScript

const timeControls = document.querySelector('.timer__controls');
const timeLeft = document.querySelector('.display__time-left');
const endTime = document.querySelector('.display__end-time');
let interval;

function secondsToHourMinSec(seconds) {
  const date = new Date(0);
  date.setSeconds(seconds);
  return date.toISOString().substr(11, 8);
}

function advanceCurrentTime(seconds) {
  const date = new Date(0);
  date.setSeconds(Math.floor(Date.now() / 1000) + seconds);
  return date.toLocaleTimeString().substr(0, 7);
}

function displayRemainingTime(seconds) {
  timeLeft.textContent = secondsToHourMinSec(seconds);
}

function displayEndTime(seconds) {
  endTime.textContent = `Be back at ${advanceCurrentTime(seconds)}`;
}

function displayTime(seconds) {
  displayRemainingTime(seconds);
  displayEndTime(seconds);

  interval = setInterval(function() {
    seconds--;
    if (seconds < 0) {
      clearInterval(interval);
      return;
    }
    displayRemainingTime(seconds);
  }, 1000);
}

function startTimer(seconds) {
  clearInterval(interval);
  displayTime(seconds);
}

timeControls.addEventListener('click', e => {
  const button = e.target.closest('button');
  if (button)
    startTimer(parseInt(button.dataset.time));
});

document.customForm.addEventListener('submit', function(e) {
  e.preventDefault();
  const value = this.minutes.value;
  if (value.length !== 0)
    startTimer(parseInt(value) * 60);
  this.reset();
});
html {
  box-sizing: border-box;
  font-size: 10px;
  background: #8e24aa;
  background: linear-gradient(45deg, #42a5f5 0%, #478ed1 50%, #0d47a1 100%);
}

*,
*:before,
*:after {
  box-sizing: inherit;
}

body {
  margin: 0;
  text-align: center;
  font-family: "Inconsolata", monospace;
}

.display__time-left {
  font-weight: 100;
  font-size: 20rem;
  margin: 0;
  color: white;
  text-shadow: 4px 4px 0 rgba(0, 0, 0, 0.05);
}

.timer {
  display: flex;
  min-height: 100vh;
  flex-direction: column;
}

.timer__controls {
  display: flex;
}

.timer__controls>* {
  flex: 1;
}

.timer__controls form {
  flex: 1;
  display: flex;
}

.timer__controls input {
  flex: 1;
  border: 0;
  padding: 2rem;
}

.timer__button {
  background: none;
  border: 0;
  cursor: pointer;
  color: white;
  font-size: 2rem;
  text-transform: uppercase;
  background: rgba(0, 0, 0, 0.1);
  border-bottom: 3px solid rgba(0, 0, 0, 0.2);
  border-right: 1px solid rgba(0, 0, 0, 0.2);
  padding: 1rem;
  font-family: "Inconsolata", monospace;
}

.timer__button:hover,
.timer__button:focus {
  background: rgba(0, 0, 0, 0.2);
  outline: 0;
}

.display {
  flex: 1;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
}

.display__end-time {
  font-size: 4rem;
  color: white;
}
<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <title>Countdown Timer</title>
  <link href="https://fonts.googleapis.com/css?family=Inconsolata" rel="stylesheet" type="text/css">
  <link rel="stylesheet" href="style.css">
</head>

<body>
  <div class="timer">
    <div class="timer__controls">
      <button data-time="20" class="timer__button">20 Secs</button>
      <button data-time="300" class="timer__button">Work 5</button>
      <button data-time="900" class="timer__button">Quick 15</button>
      <button data-time="1200" class="timer__button">Snack 20</button>
      <button data-time="3600" class="timer__button">Lunch Break</button>
      <form name="customForm" id="custom">
        <input type="text" name="minutes" placeholder="Enter Minutes">
      </form>
    </div>
    <div class="display">
      <h1 class="display__time-left"></h1>
      <p class="display__end-time"></p>
    </div>
  </div>

  <script src="https://codereview.stackexchange.com/scripts.js"></script>
</body>

</html>