var clockID = 0;

function UpdateClock() {
   if(clockID) {
      clearTimeout(clockID);
      clockID  = 0;
   }

   var tDate = new Date();

   document.getElementById('txt') .innerHTML = " Current time: " 
                                   + tDate.getHours() + ":" 
                                   + tDate.getMinutes() + ":" 
                                   + tDate.getSeconds() + "  ";
   
   clockID = setTimeout("UpdateClock()", 1000);
}
function StartClock() {
   clockID = setTimeout("UpdateClock()", 500);
}

function KillClock() {
   if(clockID) {
      clearTimeout(clockID);
      clockID  = 0;
   }
}