clock.html

<html>
<head>
<title>Clock</title>
<style>
  body {
    background-color:black;
    margin:0px;
    padding:0px;
  }
  #clockDiv {
    color:red;
    font-size:100px;
    
    text-align:center;
    width:100%;
    
    position:absolute;
    top:35%;
  }
</style>
<script type="text/javascript">
  function checkTime(i) {
    if (i<10) {
      i="0" + i;
    }
    return i;
  }
  function clock() {
    var now = new Date();
    var outStr = now.getFullYear()+'-'+(checkTime(now.getMonth()+1))+'-'+checkTime(now.getDate())+' '+checkTime(now.getHours())+':'+checkTime(now.getMinutes())+':'+checkTime(now.getSeconds());
    document.getElementById('clockDiv').innerHTML=outStr;
  }
  setInterval(clock, 1000);
</script>
</head>

<body onload="clock();">

<strong><div id="clockDiv"></div></strong>

</body>
</html>