You can extend at your local immigration office for 30 days once per entry. But since you have an METV you can just leave Thailand, reeenter and get another 60 days. You can leave and reenter up to the day your METV expires and get another 60 days. The visa is just permission to enter the country. Every entry you get 60 days. You can enter on the last valid day of your METV and get a another 60 days and then renew that for 30 days. So you can enter the last day your visa is valid and stay up to 90 days if you extend the 60 day entry at your local immigration office.
Chris Jay About six months ago my condo was raided. Many people that did have a TM30 were fined and warning notices were posted by the condo juristic person. When I came home I felt like I was back in Los Angeles with all the immigration people crawling all over the place. From what I have heard the only time they are asking for it in Bangkok is when you convert to a Non-B within country. But everything in Thailand is case by case and enforcement can change on a whim. Since the raid I keep my TM30 up to date. I got approved for the online system so I just report myself within 24 hours of arriving back in Thailand.
But you should know that immigration counts days different than almost anyone else. 15 days before is actually 14 days before using the method most people count days.