I've never been able to get a straight answer to this. Exit stamps say "admitted until...", does this mean you can leave on the date of the stamp at the latest or do you have to leave the day before the date on the stamp?
TLDR : Answer Summary
The exit stamp indicating "admitted until..." means that you can leave Thailand until the specified date. For example, if the stamp says "admitted until 09 Jun," you are allowed to fly out of the country until the end of that day, including late flights on June 9th.