Using CASE And DATEDIFF

I'm trying to create a stored procedure that will return a set of data. An input parameter (if specified) will determine what topics will be show based on the date the item "datetime".
 
ALTER PROCEDURE [dbo].[discussions_GetTopics]
@board_id as int,
@PageNumber INT,
@TopicsDays  INT = NULL,
@TopicsPerPage INT,
@HowManyProducts INT OUTPUT
AS

-- Declare the table that will store all the topics for the given board_id
DECLARE @Topics TABLE
(RowNumber INT,
topic_id INT,
topic_title VARCHAR(50),
topic_replies INT,
topic_views INT,
topic_type INT,
post_id int,
post_time DATETIME,
Topic_Author_UserName nvarchar(256),
Topic_Author_ID uniqueidentifier,
Post_Author_Username nvarchar(256),
Post_Author_ID uniqueidentifier)

-- populate the table
INSERT INTO @Topics
SELECT     ROW_NUMBER() OVER (ORDER BY discussions_topics.topic_id), discussions_Topics.topic_id, discussions_Topics.topic_title, discussions_Topics.topic_replies, discussions_Topics.topic_views, discussions_Topics.topic_type, discussions_Posts.post_id, discussions_Posts.post_time, user_1.UserName AS Topic_Author_Username,
                      user_1.UserId AS Topic_Author_ID, user_2.UserName AS Post_Author_Username, user_2.UserId AS Post_Author_ID
FROM         discussions_Topics INNER JOIN
                      discussions_Posts ON discussions_Posts.post_id = discussions_Topics.topic_last_post_id INNER JOIN
                      aspnet_Users AS user_1 ON user_1.UserId = discussions_Topics.topic_poster INNER JOIN
                      aspnet_Users AS user_2 ON user_2.UserId = discussions_Posts.poster_id
WHERE     (discussions_Topics.board_id = @board_id AND
CASE @TopicsDays
    WHEN '1' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 1
    WHEN '2' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 7
    WHEN '3' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 14
    WHEN '4' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 30
    WHEN '5' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 90
    WHEN '6' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 180
    WHEN '7' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 365
END)


-- return the total number of products using an OUTPUT variable

SELECT @HowManyProducts = COUNT(topic_id) FROM @Topics

-- Return the specified page of topics
SELECT * from @Topics
WHERE RowNumber > (@PageNumber - 1) * @TopicsPerPage
AND RowNumber <= @PageNumber * @TopicsPerPage
 
I need help where the  "CASE @TopicsDays

    WHEN '1' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 1

    WHEN '2' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 7

    WHEN '3' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 14

    WHEN '4' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 30

    WHEN '5' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 90

    WHEN '6' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 180

    WHEN '7' THEN DATEDIFF(day, discussions_Topics.topic_time, getdate()) <= 365

END" Is...   Im not doing it right. Can someone help me?