function calcage(secs, num1, num2) {
  s = ((Math.floor(secs/num1))%num2).toString();
  if (s.length < 2) s = "0" + s;
  return s;
}

function CountBack(secs, DisplayFormat, FinishMessage) {
  if (secs <= 0) {
    document.getElementById("cntdwn").innerHTML = FinishMessage;
    return;
  }
  DisplayStr = DisplayFormat.replace(/%%Z%%/g, calcage(secs,2628000,10000000));
  DisplayStr = DisplayStr.replace(/%%D%%/g, calcage(secs,86400,100000));
  DisplayStr = DisplayStr.replace(/%%H%%/g, calcage(secs,3600,24));
  DisplayStr = DisplayStr.replace(/%%M%%/g, calcage(secs,60,60));
  DisplayStr = DisplayStr.replace(/%%S%%/g, calcage(secs,1,60));

  document.getElementById("cntdwn").innerHTML = DisplayStr;
  setTimeout("CountBack(" + (secs-1) + ", '" + DisplayFormat + "', '" + FinishMessage + "')", 1000);
}

function start(TargetDate, DisplayFormat, FinishMessage) {
	var dthen = new Date(TargetDate);
	var dnow = new Date();
    ddiff = new Date(dthen - dnow);
	gsecs = Math.floor(ddiff.valueOf()/1000);
	CountBack(gsecs, DisplayFormat, FinishMessage);
}