I have a one year "B" visa. It expires on 21 July. However, returning from my last visa run, my re-entry stamp extends me to 18 August. I assume that I am cleared until 18 Aug, but not sure. Can you help me clarify this?

Second question: If I am good until 18 Aug, then may I still leave and return on a one month tourist visa, essentially extending my stay until mid-Sept?

Thanks in advance for your assistance.

It is your permisison to stay (till 18 Aug.) that is relevant. You can stay till that date.

When you leave you can return on a visa exempt entry (30 days if by air and 15 if by land border) if your nationality qualifies. With a tourist visa from a neighbouring country you would get 60 days.

Assuming you have a Multi Entry Non B Visa you can do a border run and return before 21th July and get another 90 days.

At the moment you are good until 18th August.

Thanks. Unfortunately, we are heading back home around mid-sept so no need for the 90 day extension. Thank you so much for your help. Much appreciated!

Up to you but if you are going to do a border run in August to get a Visa Exempt entry why not do a border run next week and get 90 days.

Don't forget, if you do it before the 21st you will get 90 days at a land border. If you go in August, you will only get 30 days if you enter Thailand again by air. At a land border you will than only get 15 days.