I bought a HC-EARTHERM60 in-ear thermometer a couple of years ago that is also being sold as GTE, MK-BC-T-0003, ET-100B, KN-136A, DMD1008, ... Basically it's always the same re-branded thermometer. Mine was off by nearly 10 degrees (Celsius) at some point and rather than throwing it away I opened it up to see if I could find any defect. None was found and I looked for a way to calibrate it, none was found either. So just by pushing all sorts of combinations on both available buttons I found there actually is a build in calibration function (that was accidentally triggered on mine while not in an ear, hence the huge deviation).
You need a human with an exact body temperature of 37.3 degrees Celsius (which you will probably not find). So I heated up some water mixed it with cold water until I had 37.3 degrees Celsius on a reference thermometer (kitchen thermometer for example). Dipped in the in-ear thermometer very carefully, the sensor should only touch the water surface. Now by pushing both buttons (on/recall and scan) for a couple of seconds until the thing beeps it will show 37.3 degrees and that's it. Testing again on humans resulted in values between 35.9 and 36.5 degrees Celsius rather than the disturbing 43 degrees that I had before re-calibration.