Did my local immigration office make a mistake with my visa extension dates?

if you arrived, let's say, on June 10th 2024, your 90-days stay permit got you admitted until September 9th, 2024. Any 1-year extension of stay will start on the day after the expiry of your initial 90-days stay permit. That would be September 10th 2024, and the one-year extension would be good until September 10th, 2025. Check your extension stamp, it should be accurately stamped. NOT September 1st but September 10th. Go back to your Immigration office and ask them to correct the stamp! . . .if you arrived on June 1st, 2024, then everything is correct and your EOS should expire on September 1st, 2025 . . you got a FULL year stamped

If you arrived with a non-O visa at the beginning of June, that would mean your initial 90 days would go until the beginning of September. You would apply for a 1-year extension which would go until the same date every year you apply, which would be some point at the beginning of September.

When you apply for an extension based on marriage you get the under consideration stamp, which can go past the end of your existing extension. The under consideration is then your temporary permission to stay and is an advance on your future 1-year extension which you'll receive in full after the consideration period.

Based on the times you've indicated for your initial entry and extension, it sounds normal to me. Your annual extension will always be the same date every year.

The extension will run from the date your original 90 day visa expired