I had that issue with my 13" mid-2012 MBP purchased new from an authorized reseller in 2015. That just meant that my receipt would have served as the proof of the date of purchase since apparently the reseller hadn't properly transmitted the sale information to Apple. However, that hasn't been an issue when it came to getting my MBP to work with Apple. I could get it to show up as a device on my Apple ID complete with the serial number. And recently I was able to get a battery service.
As for the warranty - Apple only has a one year worldwide warranty, so it would depend on how old it is. But if the warranty would have already expired (like any Intel Mac), it shouldn't be an issue.