Ultimately, it depends on which date Apple decides to honor if service is needed in the future.
I bought my phone on NYE last year, which is what the receipt reflects, and what most manufacturers consider the start of the warranty period.
However, I did not set up and activate the phone until a few days later, so according to the Limited Warranty section in iOS, the Estimated Expiration Date is 1/4/20, what some would call the in-service date. That is also the date reflected on Apple's My Support site, and it carries the same description.
The "Estimated" descriptor suggests that it is not considered absolute, and subject to scrutiny.
The phone was purchased directly from Apple, so it has the records of both dates in their system.
But which one will the Genius, or phone rep see, and heed if I make a request for service?
Common practice, and the legal terms side with the purchase date, but does Apple provide any leeway, and what are the bounds of any such allowances? Undoubtedly, a few days stands a greater chance of being allowed some slack than a period of a few months.
But if an item isn't a direct purchase from Apple, then the chances of any leeway decreases greatly, as the error is often in Apple's favor, not the user's. Purchases from Apple will correctly reflect the correct purchase date.
But, it is not unheard of for discrepancies to occur between the actual date of purchase, and the date reflected in Apple's records. Users can be shortchanged of warranty coverage if Apple's date is prior to the date the purchase was really made. I've had that happen with a Mac, which required going through Apple's prescribed procedure to correct the discrepancy, including the submission of a proof of purchase.
Apple has no idea when an Apple product is sold by a reseller. If a reseller fails to report, or reports incorrectly, then I suspect Apple uses the wholesale date as a default, which can be weeks or months prior. That's what I think happened with my Mac, which was NOS of the previous year's model.
Apple can be generous, but it's not legally bound to be. Erring on the side of caution would mean not sitting on the phone for months.
Edit: For more data, I just looked up the terms for the Mac I bought last month. Ordered on the 12th, picked up on the 14th, not set up until the 18th. Apple's site says coverage extends until August 13th, 2020.