PopUnder that opens every 20min per IP

I need a popunder for my site that opens every 20 min. I am using a code, but this only allows it to be opened 1 only once ...

var tags = document.getElementsByTagName("a");
for (i = 0; i < tags.length; i++ ) {
  tags[i].onclick = function(){
  if ( LerCookie('hellpop') != "1" ) {
   abreJanela();                                           
 }
GerarCookie('hellpop', '1', 1);
};

}
function abreJanela() { 
   var l = screen.availWidth; 
   var t = screen.availHeight; 
   var p = height='+t+',width='+l+',menubar=yes,toolbar=yes,location=yes,scrollbars=yes,status=yes,resizable=yes,left=0,top=0';
   nw = window.open("meu site","nt",p);
   this.focus();
}

function GerarCookie(strCookie, strValor, lngDias){
   var dtmData = new Date();

   if(lngDias){
    dtmData.setTime(dtmData.getTime() + (lngDias * 12 * 60 * 60 * 1000));
    var strExpires = "; expires=" + dtmData.toGMTString();
  }else{
    var strExpires = "";
  }
 document.cookie = strCookie + "=" + strValor + strExpires + "; path=/";
}

function LerCookie(strCookie){
var strNomeIgual = strCookie + "=";
var arrCookies = document.cookie.split(';');

for(var i = 0; i < arrCookies.length; i++)
{
    var strValorCookie = arrCookies[i];
    while(strValorCookie.charAt(0) == ' ')
    {
        strValorCookie = strValorCookie.substring(1, strValorCookie.length);
    }
    if(strValorCookie.indexOf(strNomeIgual) == 0)
    {
        return strValorCookie.substring(strNomeIgual.length, strValorCookie.length);
    }
}
return null;
}

function ExcluirCookie(strCookie){
 GerarCookie(strCookie, '', -1);
}