<div id="bb"></div>
<script>
var div=document.getElementById("bb");
 
setInterval(function(){ 
  var toDate=new Date();
  var tomorrow=new Date();
  tomorrow.setHours(17,0,0,0);
  var diffMS=tomorrow.getTime()/1000-toDate.getTime()/1000;
  var diffHr=Math.floor(diffMS/3600);
  diffMS=diffMS-diffHr*3600;
  var diffMi=Math.floor(diffMS/60);
  diffMS=diffMS-diffMi*60;
  var diffS=Math.floor(diffMS);
  var result
  result=((diffHr<10)?"0"+diffHr:diffHr);
  result+=":"+((diffMi<10)?"0"+diffMi:diffMi);
  result+=":"+((diffS<10)?"0"+diffS:diffS);
 if(diffHr>12){// 12 satten fazla varsa
    result="Süre Geçti";
 }
  div.innerHTML=result;
  
},1000);
</script>