When does the 30-day visa extension in Thailand start?

I did this yesterday actually. It went 4 days early but my extension expires 29 days from the end of the first stamp. I went to immigration division 1 and was told to go to Laksi square to do this. Was quite efficient. I printed papers from internet and filled it before going

You can usually go for your extension when you have less than 30 days left to your stamp exit date

They add on the days to your existing stamp.