Should I be concerned that my grandmother sent me a birthday card (my birthday is almost 5 months away)?

My grandmother (85 years old) has always sent me a birthday card right before my December birthday. A few days ago, however, I received a birthday card from her.

Sure, this is an easy mistake. But there is some system she has been using that has allowed her to maintain a perfect record for 40 years. But now that she’s getting older, is this just a reality or could it be a sign of something else (Alzheimer’s)?

Should I brush this off, or should I be concerned? I don’t have direct experience with this type of thing, so I am not sure how it starts.