any way to calibrate digital thermometer?

Much depends on the initial tolerance of the thermistor sensor. Typically, a 1% thermistor is good for about 0.03 C error for the thermistor alone, and about 0.5 C error if you include the associated electronics. With a cold junction reference, 0.1 C is typical. If the thermometer uses a 5% tolerance thermistor, just multiply everything by 5. Note that the OP was complaining about a 6-7 degree difference.

Details: See bottom of article for accuracy estimates.

Reminder. The goal is to measure the temperature of the air, not the room or building walls. Doing this test indoors is just asking for complications due to stratification (it's warmer near the ceiling) and the multitude of local heat sources found indoors. Isolating the temp sensor from everything except the air is why real weather stations use radiation shields.

The problem can be reduced by isolating the thermistor and using a fan to blow air to the sensor. The design turns out to be fairly complex: I'm not a big fan (pun intended) of fan aspirated radiation shields, but they do work (if you keep them clean).