There’s only one sure way to know if your bird is done: Take its temperature. It should register 165° F in the thickest part of a thigh. But if your instant-read thermometer is off, then who’s the turkey? Check its precision in a glass of water mixed with enough crushed ice to be slushy. (Be sure the tip isn’t touching the sides or the bottom of the glass.)
For a digital thermometer: For models you can recalibrate, submerge the thermometer probe in the ice water and hold down the Reset (or Calibrate) button (if it has one) or the On-Off button for 6 to 8 seconds, until the display reads 32° F.
For a dial thermometer: Submerge the thermometer probe in the ice water and, using pliers or a wrench, turn the nut just under the dial until the dial points to 32° F.