TIL a $9 oven thermometer showed me my oven runs 40 degrees cold

I bought a cheap oven thermometer at a hardware store in Tulsa last fall for about 9 bucks, mostly because my cookies kept coming out pale and my cakes always sank in the middle. I figured my oven was fine since it was only a few years old, but I stuck the thermometer in there anyway on a random Tuesday. It read 310 when I had it set to 350, and it stayed that far off the whole time it preheated. That explained so much, like why my pizza crust never got crispy and why boxed brownie mix took almost double the time on the package. Now I just bump the dial up 40 degrees and everything bakes like the recipe says it should. Best 9 dollars I ever spent on baking gear, way better than the fancy pan I bought that one time. Has anyone else found out their oven temp was way off, and did you get it fixed or just learn to adjust?