Have you ever noticed that despite what some recipes or packages declare, the time it takes to bake in your oven isn't always spot on? The issue isn't you, it's the oven itself. No matter what tricks you can think of, the issue will always be there too. It's the thermometer.
You see, oven thermometers in general can be off by 90° in either direction. It can be as little as a few degrees as well. It's never exact. Add to that the fact that ovens turn on and off to maintain temperature, you will be left with your oven's natural settings being very inexact. Over time, temperature measurements may get even more inaccurate. It's natural for a thermometer to degrade and for its accuracy to continue to diminish.
To fix the issue you will definitely need an oven thermometer that is accurate. Here's a good example of one you can find for pretty cheap. You can either note the difference between the oven's estimated temperature and the actual temperature on your purchased thermometer, or you can manually recalibrate the oven in the future. Every oven is different, so if you want to go the recalibrate route, make sure you have the oven's manual around or call an appliance repairman.
How do you know the purchased thermometer is accurate? You do the boiling water test. Place the thermometer in boiling water. At sea level it will read about 212°. The boiling point will decrease at about 1.8° for every 1000 feet increase in altitude, so make sure you know where you are and take that into account. Don't worry if it's +/- 2°, but do worry if it's +/- 20°. You'll want to seek a replacement or refund in that situation.
Either way, the moral is this: Don't always trust what your oven tells you. It could save your next batch of baked goods.