@Steven *********
, Okay, I see what you're saying, You have a current "admitted until" stamp that lets you stay until Dec 16, 2018. And if you don't exit/re-enter the country by the 13th of Sept (when your VISA expires) that's the stamp you're on and you'd apply for a yearly extension of stay at the immigration office by meeting the financail requirements and providing the required documentation>
BUT
IF you just go to the border, stamp out of thailand, stamp in and out of another country and stamp into Thailand BEFORE the 13th of Sept 2018, you'd be stamped into the country for a year from that date
Meaning you wouldn't need to apply for a yearly extension at the immigration office for another year.