Had another look at this, as the new profile page is still a ways off. I think I understand what was going on; "date added" was sort of ambiguous. When browsing product, it means "date added to the database", but it was being interpreted as "date added to collection" on the view collection page. On the edit collection page, it was falling through to "date added to the database". So, I've straightened things out:
1. added a new sort type to edit/view collection: "added to collection"
2. renamed "date added" to "added to database"
Hopefully that'll do it. I hate sorting bugs. Thanks much.