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

function CountBack(secs) {

  if (secs>0) {
    document.getElementById("CDCaption").innerHTML = "Do startu zbývá:";
  }
  else {
    document.getElementById("CDCaption").innerHTML = "Od startu uplynulo:";
    secs = -secs;
  }

  DisplayStr = DisplayFormat.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;
}

function GetDiff() {
  TargetDate = "04/20/2012 21:00";
  DisplayFormat = "%%D%% dní, %%H%%:%%M%%:%%S%% hodin.";

  var dthen = new Date(TargetDate);
  var dnow = new Date();
  ddiff = new Date(dthen-dnow);
  gsecs = Math.floor(ddiff.valueOf()/1000);
  CountBack(gsecs);
  window.setTimeout("GetDiff()",1000);
}