私が時間表示に使っているHTMLソース

↓実行結果

2000/00/00(　) 00:00:00

<script>
function clock(DOM) {
  var yobi= new Array("日","月","火","水","木","金","土");
  const timeText =
    //20XX/MM/DD(W)
           (new Date().getFullYear()) + "/"
    +("0"+ (new Date().getMonth()+1)).slice(-2) + "/"
    +("0"+  new Date().getDate()    ).slice(-2) + "("
    +(yobi[ new Date().getDay() ]   ) + ")"
    +" "
    //時:分:秒
    +("0"+new Date().getHours()  ).slice(-2) + ":"
    +("0"+new Date().getMinutes()).slice(-2) + ":"
    +("0"+new Date().getSeconds()).slice(-2)
  ;
  if(DOM){document.getElementById("viewClock").innerHTML = timeText;}
  else{return timeText;}
}
window.onload = function(){clock("DOM"); console.log(clock());}
setInterval('clock("DOM")',1000);
</script>
<div id="viewClock">2000/00/00(　) 00:00:00</div>