Does leaving Thailand reset my 90-day report clock?

The word doing the work in the rule is consecutive. The 90-day report is owed after 90 unbroken days of presence, so a real departure, of any length, breaks the run. The Thaiger's reporting guide puts it plainly: departure and re-entry ordinarily end the current cycle and start a new one from the new arrival date.

What the new date is

Your next due date is 90 days from the entry stamp, not from the flight home and not from your last report. If you already had a report on file, the old due date simply falls away.

Where people get caught

1. Counting from the last report rather than the last entry. After a trip abroad, the entry stamp is the only date that matters.
2. Leaving to escape a late report. The same guide warns that departure does not retroactively erase a violation that was already complete before you left. If the report was overdue on the day you flew, it was overdue.
3. Assuming a re-entry cancels the requirement for good. It restarts it, and a long unbroken stretch afterwards brings it back. On a DTV, where one entry allows up to 180 days, most holders who stay put will reach it.

The clock that can break instead

Your permission to stay is a separate matter, and leaving can end it rather than reset it. ThaiLawOnline explains that on a single entry visa or an extension, getting stamped out without a re-entry permit terminates it immediately. Multiple entry visas, including the DTV, the LTR and the METV, do not need one.

And the tax clock, which does not reset

Days out of Thailand do not erase days already spent in it. Thai tax residency adds up all your days in the country from 1 January to 31 December, consecutive or not, so a week abroad takes seven days off the total and nothing more.

Filing itself, online or at the counter, is covered in what the 90-day report is and how to file it online, and the full set of staying legal rules is in our guide to arriving and staying legal in Thailand. Count your days with Leave By Check.