var timerID = null;
var timerRunning = false;

function stopclock (){
  if(timerRunning)
    clearTimeout(timerID);
  timerRunning = false;
}

function startclock () {
  stopclock();
  showtime();
}

function showtime () {
  var now = new Date();
  var hours = now.getHours();
  var minutes = now.getMinutes();
  var seconds = now.getSeconds();
  var timeValue = "" + ((hours >12) ? hours -12 :hours);
  
  timeValue += ((minutes < 10) ? ":0" : ":") + minutes;
  timeValue += ((seconds < 10) ? ":0" : ":") + seconds;
  timeValue += (hours >= 12) ? " P.M." : " A.M.";
  
  document.getElementById("clock").value=timeValue;
  
  timerID = setTimeout("showtime()",1000);
  timerRunning = true;
}

startclock();