linq内部不支持AddDays之类的运算，不过没关系，把它放到外部就可以了

        Dim time1 = DateTime.Now.AddDays(-30)
        Dim topics = (From topic In db.Topics Where topic.CreatedAt > time1 Select topic Order By topic.ID Descending).ToPagedList(pageNumber, 10)

就这么简单