Will AppleCare+ officially start the date of purchase of the AppleCare+ OR the iPhone?

I'm glad you asked for clarification and indeed I have now learned I was wrong and Apple was correct. Contained in this link, in the little print for footnote #3 is the following:

Coverage begins on your AppleCare+ purchase date. Any unused incidents will expire after 12 months of coverage and you will get two more incidents to use within the next 12 months of continued coverage.

And the link this came from is this --> AppleCare Products - iPhone - Apple

It is tied to the date the phone was purchased, regardless of the date you add the plan.

To be accurate, the plan actually states:

Plan coverage begins when you purchase the Plan and continues, unless cancelled, through the date specified in your Plan Confirmation (the “Plan Term”).

https://www.apple.com/legal/sales-support/applecare/applecareplus/0921/091421_applecareplus_us.pdf

In short, the plan begins at purchase, but Apple dictates the end date in your plan documents. Their document does not state whether that is typically a period from device purchase or plan purchase, it simply states the plan ends when your Apple plan document says it does.

For me it's the same as my delivery date plus two years, but I purchased my plan with my device.

Only someone who purchased within the 60 day post-purchase window could comment on where their end date falls, and I suspect it varies from state to state and country to country.

Date of purchase of the phone.

It's an extended warranty.