depends on the office you go to some allow the extension early some only allow the extension 7 days or less of the expiry of the 30 days. The 30 days gets added to the original expiry date not from day of application for extension
Sounds like your arriving 30 days visa exempt just an arrival stamp in passport. This you get every time you leave and arrive but can be extended at an immigration office for another 30 days.
You can do 2 land entrees and get 30 day stamp exempt visa and no limit by air until an IO decides you need a proper visa to visit again.