I thought you used to have a year to purchase it. Was going to purchase it on my wife’s new MacBook Air and realized that I had neglected to buy it for an iMac purchase in the early spring. According to Apple’s website, it said this computer was eligible but when I called support they said no. Am I misremembering this policy, or did it change recently?
It changed when they included accidental damage insurance.
You might want to call Apple Support again and push them on it. I bought a MacBook Pro in February 2017 and waited to get AppleCare, thinking I had a year to do it, but they changed the window a few months later. So when I finally called in January 2018, the first person I spoke to said I was out of the window and couldn’t get it, even though the website said my MBP was eligible. It took escalating it to a manager before they finally let me get AppleCare under the old one year deadline. I can’t remember when they changed it, but I want to say it was sometime between April 2017 and WWDC that year.
So depending on when exactly they changed the policy and when exactly you bought your iMac, you might still be able to wrangle it.
Computers bought after July 2017 have a 60 day window to purchase AppleCare+ now. Unit bought before then were only able to add AppleCare within the one year purchase date. Outside of that timeline and you are out of luck.
We used to joke that not even Steve Jobs could purchase AppleCare past that timeline.