So I've read a few threads here on calibrating digital Thermometers using the 212* boiling point of water as an accurate gauge. The boiling point of water is 100°C or 212° F at 1 atmosphere of pressure (sea level). So even at sea level there will be a corrected temp depending on the barometric pressure. I don't want to bore anyone with the formula and all the other mombo jombo that goes with it. There is an online calculator one can use. You need to know your altitude and the barometric pressure. I have a home weather station that gives me all the info. If you do not you can use a gps to get elevation, or if you have an airport nearby you should be able to get elevation and the barometric pressure from their website.
For example the altitude where I live is 3640 and today's barometric pressure is 26.79 inches HG. the boiling point for water today is 205.46
My digital therm read 199 today in boiling water. Six degrees off, which is where it's been since the day I've owned it. I periodically test it just to make sure.
Here's a link to the online calculator: