This post includes thoughts...without necessarily answers.
If my daughter tells someone her date of birth, she is a secondary source of that date. She has no first hand knowledge of her date of birth.
If I tell someone that today is my daughter's 21st birthday (which it isn't, but pretend that it is), is that secondary? I was present at the birth, but if I say it or write it down 21 years later is that record primary or secondary? If I write it down with a month of her birth, that probably would be considered primary. But what about 21 years after the fact, even if I had first hand knowledge of the event?