What is the correct procedure for my 90-day visa report in Thailand, and does reporting online affect my extension?

Immigration offices count days usually including the day of origin. You can try your 90 days report online on and after 31st August until 14th September.

IO meant you CAN report once you pass 15 days before your due date, not you MUST report 15 days in advance.

If you report 90 days earlier, your next 90 days report due is the 90th day from the reporting date. and it would never affect your extension even if you report earlier or later, your extension expiry does stay the same. Go online and try reporting when 15 days before your due date.

Your report may or may not be approved on the first try. If rejected you would still have some time to retry even a few times more before you have to go to the immigration office to report in person again.

90days report windows are:

- At the local immigration office: 15 days before until 7 days after the due date

- Online: 15 days before until the due date

- By legacy mail (EMS from Thailand Post): post stamped at least 15 days before the due date