<header>
  <h2 class="title">Date.now()</h2>
  <p class="description">Отримання поточного часового штампу в мілісекундах.</p>
</header>
<main>
  <div class="result">
    <button id="startBtn">Старт</button>
    <p>Минуло часу: <span id="elapsedTime">0</span> секунд</p>
  </div>
</main>

body {
  font-size: 16px;
  line-height: 1.5;
  font-family: monospace;
}

header {
  background-color: #f1f1f1;
  margin-bottom: 25px;
  padding: 15px;
  -webkit-box-shadow: 0px 0px 3px 0px rgba(118, 118, 118, 1);
  -moz-box-shadow: 0px 0px 3px 0px rgba(118, 118, 118, 1);
  box-shadow: 0px 0px 3px 0px rgba(118, 118, 118, 1);
}

header h2.title {
  padding-bottom: 15px;
  border-bottom: 1px solid #999;
}

header p.description {
  font-style: italic;
  color: #222;
}

.result {
  background-color: #f8f8f8;
  padding: 15px;
  -webkit-box-shadow: 0px 0px 3px 0px rgba(118, 118, 118, 1);
  -moz-box-shadow: 0px 0px 3px 0px rgba(118, 118, 118, 1);
  box-shadow: 0px 0px 3px 0px rgba(118, 118, 118, 1);
}

button {
  padding: 10px 15px;
  margin-bottom: 15px;
  cursor: pointer;
}

const startBtn = document.getElementById('startBtn');
const elapsedTime = document.getElementById('elapsedTime');

let startTime = 0;
let interval;

startBtn.addEventListener('click', function() {
  if (startTime === 0) {
    startTime = Date.now();
    interval = setInterval(updateTime, 1000);
    startBtn.innerText = 'Зупинити';
  } else {
    clearInterval(interval);
    startTime = 0;
    elapsedTime.innerText = '0';
    startBtn.innerText = 'Старт';
  }
});

function updateTime() {
  const now = Date.now();
  const secondsPassed = Math.floor((now - startTime) / 1000);
  elapsedTime.innerText = secondsPassed;
}

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External JavaScript

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