Original TR visa 60 day entry stamp is December 18, expires February 17. Went yesterday, Chiang Mai immigration office by the airport to find out our options. Was told to come back January 29 for covid extension because it would start effective the date we apply and NOT the date our existing stamp runs out. So in our case, by getting the 60 day covid extension on the last day possible, January 29, we'd only gain 12 days over our normal 60+30 TR length. We've heard other people have gotten the stamp in addition to time remaining from the same office, but can't confirm what their visa situation was. Really hoping they extend the January 29 covid extension deadline!