In this list you will see many examples of time turning what was a secondary document into a primary document. Yes it can be confusing. For more thorough-going examples and definitions see the Yale University site below.
Yale University has a very well-designed page describing primary sources. While all of the Suggested tools for... lead to Yale owned material, the examples/definitions for each type of material are excellent.
The list below have all been reviewed as either a Choice Outstanding site or a Choice Highly Recommended site.