<body onload="startTime()">
<div id="time"></div>
body {
  display: flex;
  height: 100vh;  
  align-items: center;
  justify-content: center;
 }

#time {
   position: relative;
   color: #333;
   font-size: 50px;
   font-family: Impact, Charcoal, sans-serif;
   letter-spacing: 10px;
}
function startTime() {
  var today = new Date();
  var h = today.getHours();
  var m = today.getMinutes();
  var s = today.getSeconds();
  m = checkTime(m);
  s = checkTime(s);
  document.getElementById('time').innerHTML =
  h + ":" + m + ":" + s;
  var t = setTimeout(startTime, 500);
}
function checkTime(i) {
  if (i < 10) {i = "0" + i};  
  return i;
}
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