Can I apply for a 30-day extension after my tourist visa expires, and how is the extension period calculated?

bring coffee money

You can go up to 30 days before it expires and you will not lose any time.

One can submit ..Within 1 week before expiry I have experienced before !

as you're asking the question , do you think someone is gonna say " oh no problem one day late , ,,mai pen rai !!!" ??..

Although in theory you can wait until Monday (as the office is closed on the Sunday), but you will get a small stamp indicating one day overstay - fine waived due to office closed. As you have 30 clear days before to get the extension, I agree with

@Brandon ***********

. Do it beforehand

The expiration of your tourist visa doesn't matter. It's the date on the stamp you received when you entered Thailand that matters.

At most immigration offices you can apply for extension when you have 30 days or less left on your entrance stamp. At Chiang Mai you can apply when you have 45 days left. So you can apply 15 days after entering the country on a tourist visa. The 30 days would be added to the end of your existing stay, meaning you would not lose any time. There is no reason to wait until the last minute to apply.