javascript cookie 有效时间一个月怎么写？

Question

//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？