Will my 30-day extension in Thailand start on the day I apply or the expiration date of my current visa exemption?

People always seem to wait until the last second to apply for a 30 day extension on tourist visas and 30 day visa exempt entry stamps.

Believe me there is NO reason to do that. They ADD the new extension to your current expiration date so you don't lose any days by going early.

Also IF your current stamp expires on a day the immigration office is closed (weekends/holidays) and you go the very next day they're open you can get your extension like normal.

Keep in mind you will get an overstay stamp in your passport, but you will not be fined the 500baht overstay fine. It will say fine waived because of holiday.

They would add - top up - 30 days. But you just need to check you got additional 30days when you receive your passport. Don't wait until the last minute, anything would gonna come up.

They'll give you the extra days on top of your already admitted days. So in your case, they'll add 30 days to the 21st