I agree with Maccotto, 60 days after purchase to buy AppleCare+.
Old AppleCare is not available online, but I have heard you can call Apple and purchase it. The old AppleCare has to be purchased within 1 year. I would caution that I have not personally called Apple to try and purchase the old AppleCare so you might want to call Apple to verify it's availability before 60 days passes.
You have a choice. You can purchase AppleCare+ within 60 days of purchase, or rely on what somebody tells you on the telephone, assuming that they tell you what you want to hear, and wait a year.
In my view, not a hard decision.
I purchased AppleCare+ a couple of days ago at day 50 from purchase. It's good until 9 January 2022 and, at US$99, will cost me 9 cents per day.
So AppleCare runs from the actual date you buy AppleCare and Not the Day you bought your Hardware?
So for me I purchased AppleCare when I purchased my Mini-Dec 10-so AppleCare for me ends Dec 10 2021?
If I would have known I would have waited like you!!