The activation date does not match

That happens when you buy from someone who is neither Apple nor an Apple-authorized reseller. Independents order Apple products like an individual would, and the warranty clock starts when the device leaves Apple control.

This is a very common question here, especially from countries and regions that do not have an official Apple presence, or have laws restricting foreign-owned businesses.

It sometimes happens that the date differs from the purchase date. I have tried that before. Sometimes you can enter the purchase date yourself here mysupport.apple.com, but most often you have to contact Apple Support to have them set the correct date.

You can find more information here:

Update your proof of purchase - Apple Support

Just wanted to add. That's absolutely true - I almost always find that the date in Settings > General > About doesn't match the purchase date. I often buy Apple equipment in my local electronics stores, as we don't have an Apple Store in Denmark. Conversely, when I buy equipment directly through Apple (via their homepage), the date always matches perfectly.

Thanks for enlightening me on this. It makes sense.

The warranty period starts the day the device is sold by Apple or an authorized retailer. When it's actually activated is irrelevant.