Why do you use new Date instead of a static UTC string?
function clearListCookies(){
var cookies = document.cookie.split(";");
for (var i = 0; i < cookies.length; i++){
var spcook = cookies[i].split("=");
document.cookie = spcook[0] + "=;expires=Thu, 21 Sep 1979 00:00:01 UTC;";
}
}