What is a "certificate of authenticity" and what is it's significance? Who issues these?
It is simply a certificate that says the object you hold is authentic, and not a fake or misrepresented.
The certificate can be issued by anyone who is knowledgeable enough to tell you if the item is real or not. And then the strength of the certificate would depend on who issues it.
I don't know why time is more important than proving it is an authentic coin, straight from Mike and not loaded by someone else. You know what year is on the coin, and you can see in the blockchain when the bitcoins were sent to that address. What else matters?
The problem is counterfeiting. How do you know that the coin you are buying is genuine, short of scratching off the hologram to check the private key?
Molecular's idea is excellent because it allows a photograph to be dated. If the coin being sold matches the photograph then you know that it was in existence at the date of the photograph. This does not establish authenticity, but the older the coin the less likely to have been counterfeited.
. Please do. 
