melting ice (not ice with water added) is the low level calibration for 32 degrees F (0 degree C) and an open container of water at a rolling boil is the the high level calibration point of 212 degree F (100 degree C).
These are industrial plant standards and acceptable for calibration purposes.
As an aside, I have never had a butt that went for more than 14 hours to reach the proper internal of 195 as long as the smoker temp is held between 225 and 250 and have cooked them in less than 8 hours with the use of a 'Texas' crutch (aluminum foil) after four hours of smoke. Higher temps will shorten the time for the cook. I would question a lot of things with 21 hours of cook time.