Mettre un countdown sur mon site web

Bonjour,
tous le monde je suis ici pour vous demander un petit service ;) je code mon site web mais je ne sais pas comment faire un countdowns mais qu'a la fin s'a affiche un truc comme par exemple une vidéo youtube mes code sont la :

HTML :

Code:

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</pre>
<h1><font face="Agent Orange"><font color="#000000"><font size="5">
	Ma nouvelle video sors dans :
</h1>
<span><font size="7"><font face="Agent Orange"><font color="#000000"> <div align="center">
<center><script language="JavaScript">
TargetDate = "12/7/2020 11:16 PM";
BackColor = false;
ForeColor = false;
CountActive = true;
CountStepper = -1;
LeadingZero = true;
DisplayFormat = "%%D%%J&nbsp;:%%H%%H&nbsp;:%%M%%M&nbsp;:%%S%%S";
FinishMessage = "Ma nouvelle video est sortie";
</script>
<script language="JavaScript" src="compteur.js">
</script>

JS :

Code:

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function calcage(secs, num1, num2) {
  s = ((Math.floor(secs / num1)) % num2).toString();
  if (LeadingZero && s.length < 2)
    s = "0" + s;
  return "<b>" + s + "</b>";
}
 
function CountBack(secs) {
  if (secs < 0) {
    document.getElementById("cntdwn").innerHTML = FinishMessage;
    return;
  }
  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;
  if (CountActive)
    setTimeout("CountBack(" + (secs + CountStepper) + ")", SetTimeOutPeriod);
}
 
function putspan(backcolor, forecolor) {
  document.write("<span id='cntdwn' style='background-color:" + backcolor +
    "; color:" + forecolor + "'></span>");
}
if (typeof(BackColor) == "undefined")
  BackColor = "white";
if (typeof(ForeColor) == "undefined")
  ForeColor = "black";
if (typeof(TargetDate) == "undefined")
  TargetDate = "12/31/2020 5:00 AM";
if (typeof(DisplayFormat) == "undefined")
  DisplayFormat = "%%D%% Days, %%H%% Hours, %%M%% Minutes, %%S%% Seconds.";
if (typeof(CountActive) == "undefined")
  CountActive = true;
if (typeof(FinishMessage) == "undefined")
  FinishMessage = "";
if (typeof(CountStepper) != "number")
  CountStepper = -1;
if (typeof(LeadingZero) == "undefined")
  LeadingZero = true;
CountStepper = Math.ceil(CountStepper);
if (CountStepper == 0)
  CountActive = false;
var SetTimeOutPeriod = (Math.abs(CountStepper) - 1) * 1000 + 990;
putspan(BackColor, ForeColor);
var dthen = new Date(TargetDate);
var dnow = new Date();
if (CountStepper > 0)
  ddiff = new Date(dnow - dthen);
else
  ddiff = new Date(dthen - dnow);
gsecs = Math.floor(ddiff.valueOf() / 1000);
CountBack(gsecs);

voilà merci de m'aider merci ;)