<script>
function display(hformat) {
 var thedate = new Date();
 var hours = thedate.getHours();
 var Min = thedate.getMinutes();
 var Sec = thedate.getSeconds();
 if (hformat == 24) {
  var TimeV = "" + hours
 } else {
  var TimeV = "" + ((hours >12) ? hours -12 :hours)
 }
 
 TimeV += ((Min < 10) ? ":0" : ":") + Min
 TimeV += ((Sec < 10) ? ":0" : ":") + Sec
 if (hformat != 24) {
  TimeV += (hours >= 12) ? " P.M." : " A.M."
 }
 
 window.status = TimeV;
 
 str = "display("+hformat+")"
 timerID = setTimeout(str,1000); 
}
display(12);
</script>

Demó:

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