Can I leave Thailand on the 9th if my entry stamp says I can stay until that date?

As you ask in a DTV group, I assume you have a DTV.

OR you leave Thailand, by air, land or sea before midnight 9 march,

OR visit a local Immigration Office to apply for a 180 day Extension of Stay

If you have another visa type the length of Extension of Stay will differ as it depends on used visa on entry.

23:59 on the 9th

At the moment, you can get "emergency extension" if your flight have been cancelled, and overstays dont lead to actions as long as you leave "as soon as possible". That was off topic, the stamp says the day you have to leave

As the others said, 9th, but scheduling your exit for the 9th doesn't leave room for any contingencies. It's not usually a catastrophe if something happens, usually only a 500B/day fine, but you might want to consider scheduling for earlier just in case. If you actually leave on the 9th you're fine though.

You are allowed to still untill the 9th, so that’s the day you need to leave

9 th