I've had to contact Apple for a warranty start date adjustment for some (but not all) Macs I've bought from third party resellers. Each time, it was painless—explained the problem in a support chat with Apple, they ask me to upload a copy of my invoice showing the purchase date, warranty adjusted, the end. It's not that big of a deal.
It all depends when Apple considers the device sold—many resellers drop-ship direct from Apple or its distributors. In that case, Apple has direct knowledge of when a device is sold since it doesn't leave their control until the purchase is completed. Large resellers, like Best Buy or Amazon, are going to buy huge lots from Apple at negotiated prices, especially this time of year. Apple will consider those devices sold at that point, versus when the customer takes ownership.
In the end, it's accounting rules that govern all this. Sorry you had to go through a bit of a rigamarole, OP. Hasn't generally been my experience.
