How do I calibrate a thermometer?

I have a metal thermometer that I want to use in my darkroom. The temperature reading is off by about 20 degrees. Isn't there some way to adjust it by using a glass of ice water?

Would a kitchen thermometer work as a darkroom thermometer?

I’m developing film tomorrow in class and i forget to get a darkroom thermometer and all the photo stores are closed, would a kitchen one work, granted i don’t use it for food ever again?
Yep, it’s for B&W.