javascript cookie 有效时间一个月怎么写？

Question

//设置cookiecookie.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？