TSOW thanks for asking about this, it would be good to have a tool like this, as sometimes even now being on everyday and using the tools here I still end up missing some threads and posts and I usually see some when the newsletter arrives!
One drawback to the add-on I found is that it requires that the record of which threads are read and unread be stored in the database.
From an administrative point of view, this is an issue for two reasons:
1. It adds to the size of the database, making both backups lengthier and increasing the risk of issues if you ever have to transfer your website to another server; and
2. It means extra database queries every time anyone accesses a thread or does a search for new posts - on some servers, this may create the kind of databasae errors we saw frequently on our old server.
i think the best thing for a user to do would be to do a search and input the dates for which they want to find threads. i believe there is an option in the advanced search screen to specify dates for posts.