Great question and let me tell you all about my experience. I bought an iPhone XS Max and Apple Watch and checked the dates to see when AppleCare ended. It was the day my credit card was charged. So I was looking at this:
Phone shows 9/18/20 end date instead of 9/21/20 which was date received
Watch shows 9/15/20 end date instead of 10/8/20 which was date received (almost a month burned off before getting the device)
Between yesterday and today I've spent a bit over an hour working to get the end dates corrected to when I actually received them. It is not right that Apple starts AppleCare when your card gets charged. In fact, there is a firm looking to do a class action about this.
I did get the dates corrected and encourage everyone who buys AppleCare at pre-order time to call (don't chat - they can't help).