Nu exista functia readCookie, dar cu un mic search am gasit ceva ce te-ar putea ajuta:
Cod:
function getCookies(){
    var myCookies = {};
    var cookies = document.cookie.split(';');
    
    for (var i = 0; i < cookies.length; i++) {
        var crumbs = cookies[i].split('=');
        var name = unescape(crumbs[0]).replace(/^\s+|\s+$/g, ''); // trim any whitespace
        var value = unescape(crumbs[1]).replace(/^\s+|\s+$/g, ''); // trim any whitespace
        
        myCookies[name] = value;
    }
    
    return myCookies;
}

var cookies = getCookies();
//Aici implementarea pe cazul tau

if (!cookies['cookie']) {
					setTimeout('hide()',10*1000); // Time Setting
					function hide() {
					var x=document.getElementById('a').style;
					var y=document.getElementById('b').style;
				
				if(x.display=='block') { x.display='none'; y.display='block'; cookie('cookie','1',1);}
					else { x.display='block'; y.display='none'; }
		}
	} else {
		var x=document.getElementById('a').style;
					var y=document.getElementById('b').style;
		 x.display='none'; y.display='block';
		}