var t=new Date();
var d=new Date("October 31, "+t.getFullYear());
var diff = Math.ceil((d.getTime()-t.getTime())/86400000);

var hmsg = "It's Halloween!";

if(diff<0){
  d    = new Date("October 31, "+(t.getFullYear()+1));
  diff = Math.ceil((d.getTime()-t.getTime())/86400000);
}

if(diff>0){
  hmsg = diff + " Days 'til Halloween";
}
document.write(hmsg);