ちょうど最近書いたものがありました。必要なところを適当に抜粋しています。
<html>
<head>
<meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
<script type="text/javascript">
setInterval("WriteDate()",1000);
function WriteDate(){
var myMonthName=new Array("Jan.","Feb.","Mar.","Apr.","May","Jun.","Jul.","Aug.","Sep.","Oct.","Nov.","Dec.");//月名称定義
var myWeekTbl = new Array("SUN","MON","TUE","WED","THE","FRI","SAT"); // 曜日テーブル定義
var objDate=new Date();
var objJDate=document.getElementById("JapanDate");
var localTime=objDate.getTimezoneOffset()/60;
var japanTime=-9;
var timeDeff=(japanTime-localTime)36001000;
objDate.setTime(objDate.getTime()-timeDeff);
mon=myMonthName[objDate.getMonth()];
date=objDate.getDate();
day=myWeekTbl[objDate.getDay()];
hh=objDate.getHours();
mm=objDate.getMinutes();
ss=objDate.getSeconds();
objJDate.innerText=day + " " + mon + date + " " + hh + ":" + mm + ":" + ss ;
}
</script>
</head>
<body>
<DIV id="JapanDate" style="text-align:center;background-color:red;color:white"></DIV>
</body></html>
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