var CookieUtil = {

    get: function (name){
        var cookieName = encodeURIComponent(name) + "=",
            cookieStart = document.cookie.indexOf(cookieName),
            cookieValue = null,
            cookieEnd;
            
        if (cookieStart > -1){
            cookieEnd = document.cookie.indexOf(";", cookieStart);
            if (cookieEnd == -1){
                cookieEnd = document.cookie.length;
            }
            cookieValue = decodeURIComponent(document.cookie.substring(cookieStart + cookieName.length, cookieEnd));
        } 

        return cookieValue;
    },
    
    set: function (name, value, expires, path, domain, secure) {
        var cookieText = encodeURIComponent(name) + "=" + encodeURIComponent(value);
    
        if (expires instanceof Date) {
            cookieText += "; expires=" + expires.toGMTString();
        }
    
        if (path) {
            cookieText += "; path=" + path;
        }
    
        if (domain) {
            cookieText += "; domain=" + domain;
        }
    
        if (secure) {
            cookieText += "; secure";
        }
    
        document.cookie = cookieText;
    },
    
    unset: function (name, path, domain, secure){
        this.set(name, "", new Date(0), path, domain, secure);
    }

};

//设置cookie
cookie.set("name","nicholas");
cookie.set("book","Professional Javascrpit");

//读取cookie的值
cookie.get("name");
cookie.get("book");

cookie.set("name","NIcholas","/books/projs/","www.wrox.com",new Date("2016,8,28"));
请问我想设置 cookie有效时间1个月怎么做啊？
　而不是通过日期，比如30 啥弄的？