MySQL get dates that have records but are missing a specific record

Hopefully the title isn't too vague, but I'll explain. I'm pretty new to advanced queries so unfortunately this is not as much of a "i'm trying this" as much as a "how would I even go about doing this" type of question.

I have a table of user activity. When they do something on the site, it gets entered into this database. Unfortunately there was a change in how logging-in was tracked. So now I need to find all records for each user id, check the date for each record and see if there is a record with a specific id for that date.

I realize that sounds confusing so I will break it down into the best programming logic I can

1. get list of records for user_id order by date
2. for each date, check if id 5 exists
3. if id 5 doesn't exist, tell me about

Basically I will be inserting that id into the table where that id doesn't exist based on that date.

Any help would be greatly appreciated. Thanks in advance.

Edit**I forgot to add a basic structure here.

id      | user_id      | item_id     | stamp------------------------------------------------------1       | user1        | 1           | 2012-08-252       | user1        | 2           | 2012-08-253       | user1        | 3           | 2012-08-254       | user1        | 4           | 2012-08-255       | user1        | 6           | 2012-08-256       | user1        | 7           | 2012-08-257       | user1        | 5           | 2012-08-268       | user1        | 11          | 2012-08-269       | user1        | 13          | 2012-08-26

So that's the basic structure. User1 is missing id 5 on 2012-08-25 so I would need to add it in with that date. user1 is not missing id 5 on 2012-08-26 so I would not need to insert anything at all. Does that clear anything up?