Do I need to account for previous days in Thailand when re-entering for 90 days?

What you posted is the law. Yes.

90 days within 6 months max. In one or multiple pieces.

But it isn’t enforced right now.

Many do 60+30 then a few weeks outside and then again.

Some have been warned not to do that again, some have been denied when trying a third time.

So with your 12 days only, you’re fine.

If you want to do more than 6 or 9 months in a calander year without spending a few weeks in another country to look like a tourist, take a Thai boxing or Thai cooking class that will give you the letter for a DTV visa. You can visit all year for 5 years on that with mutiple 180 day extension

this rule isnt strictly enforced, its a case by case situation

Just like in PH it resets each time you leave. So another 60 days visa free plus you can extend.

But unlike PH you can’t extend for years without leaving and officers will warn or even deny entry if they think you are living in Thailand on visa free entry.

There are ways around that with services but PH is way easier when it comes to immigration rules.

So if you plan to switch to living in TH look into a real visa. But just for vacation trips you will be fine.

Just be aware the longer and more often you stay the harder it is to get in without help from a service.

12 days and then coming in for 60+30 will be fine. But leaving for a day and coming right back after that might have issues. Even leaving for a month and coming back for another 60+30 may. And by the 3rd time in a row (with only smaller times outside of TH) the risk goes up a lot more.

Some do it for years and never have an issue. Others will get a more serious talk by the 2nd or 3rd time in a row. It just is a toss up.

So again don’t treat it like PH and expect to just keep going in and out forever. Get a real visa if you want to stay long term.

Beyond that enjoy a long stay vacation in TH. You should have no issues.

Anyone know the requirements for the airlines coming in on a 60 day stamp ? Can he show return 89 days later ?

There is a limit to how many 30 extensions

There’s no official limit or regulations regarding this and everything is in the discretion of the IO you’re standing in front of. If they mean that you aren’t a genuine tourist but use tourist solutions for long term stay in Thailand you risk to get pulled aside and told to get a proper visa according to your travel pattern and/or denied entry.

If you only stayed for twelve days you should be able to re-enter as a genuine tourist and get another 60 days visa exemption and do a 30 days extension after. No subtraction is (or can be) done.

60+30 each trip. If you’ve been there already 12 days, it’s 60-12 plus the 30 day extension. If you’ve been there only for 12 days then left and come back, you get a new 60+30

No, as long as you don’t abuse the system, you’ll get 60+30 entering as an American.

No. Once you have left Thailand, you get a fresh 60 days when you go back to Thailand.