Since temperatures are critical in brewing, it's important that your thermometer tells you the truth. Many of us (including me) blindly follow what these things tell us, but you'd be surprised how often they are wrong.
This video shows you the right way to make an ice bath so you can calibrate your thermometer.
It's intended for the food services industry but is also useful for homebrewers. In fact, Thermoworks products are becoming widespread in the homebrewing market. I'm a happy customer myself.
As an example of how it important it is to have an accurate thermometer, say you are conducting a single infusion mash and shooting for a temperature of 150°F. If your thermometer reads 7°F on the low side, you'll actually be mashing at 157°F which will give you a less fermentable wort and much different beer.
Not an all-grain brewer? What if you were steeping your grains above the dreaded 170°F and didn't even know it? Or if you adjusted for the wrong temperature when taking a gravity reading?
Even if your thermometer can't be calibrated, at least you can know how much you are off by (though it could change at different temperatures).
Time to be honest – have you ever checked the accuracy of your homebrewing thermometer?