javaScript显示日期，自动计时

<script>
  <!--var day="";
  var month="";
  var ampm="";
  var ampmhour="";
  var myweekday="";
  var year="";
  mydate=new Date();
  myweekday=mydate.getDay();
  mymonth=mydate.getMonth()+1;
  myday= mydate.getDate();
  year= mydate.getFullYear();
  if(myweekday == 0)
  weekday=" 星期日 ";
  else if(myweekday == 1)
  weekday=" 星期一 ";
  else if(myweekday == 2)
  weekday=" 星期二 ";
  else if(myweekday == 3)
  weekday=" 星期三 ";
  else if(myweekday == 4)
  weekday=" 星期四 ";
  else if(myweekday == 5)
  weekday=" 星期五 ";
  else if(myweekday == 6)
  weekday=" 星期六 ";
  document.write(year+"年"+mymonth+"月"+myday+"日 "+weekday);
  //-->
   </script>