HTML5 localStorage timer

<!DOCTYPE html>
<html>

<head>
  <link rel="stylesheet" href="style.css">
  <script src="script.js"></script>
</head>

<body>
  <h1>HTML5 local storage timer</h1>
  <p>Uses HTML5 localStorage to record the first time this page is loaded, and counts the seconds thereafter</p>
  <div id="timerLabel">Loading...</div>
  <button onclick="resetStartTime()">Reset</button>
</body>

</html>

// Code goes here
function resetStartTime() {
  startTime = new Date();
  window.localStorage.setItem('startTime', startTime);
  return startTime;
}
document.addEventListener('DOMContentLoaded', function(event) { 
  // get timestamp
  startTime = new Date(window.localStorage.getItem('startTime') || resetStartTime());
  // start timer
  window.setInterval(function() {
    var secsDiff = new Date().getTime() - startTime.getTime();
    document.getElementById('timerLabel').innerText = Math.floor(secsDiff / 1000) + ' seconds elapsed.';
  }, 1000);
});

/* Styles go here */