I'm on a 14" M1 Max from 12/2021 and my AC+ expires in 2025. I'm pretty sure you can buy it right before your warranty expires to extend it. But maybe you bought 1 year of AC+ and can't extend it after?

This was my first AC+ purchase.

Also my battery has been at 79% Service Battery health for about a year. I forgot to use AlDente to limit the battery to 80% but do it now and it hasn't gone down.

edit: Jesus I just realized 2025 is next year. Totally thought I had like 3 years left.. nevermind..