My 90 days on DTV ends on 22 Dec. I have a flight on 27 Dec. I understand there's a window of 7 days after, by when I need to report?
would I need to do so given this schedule?
TLDR : Answer Summary
Yes, you need to file your 90-day immigration report in Thailand before you leave, even if your flight is on December 27. The reporting window allows you to report late, but does not exempt you from reporting altogether. You can report your status starting weeks before your due date.
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