I saw this question posted to ELECTRONICS HOBBY CIRCUITS IN FACEBOOK group:
“can you tell me what is the meaning of calibration date of a multimeter and does it effect the work of a multimeter if it expires the calibration date?”
The answer to this is:
As all circuits will drift with time, the DMM will need to be periodically re-calibrated to ensure that it is operating within its specification. The calibration period will form part of the specification for the DMM.
The calibration includes verification that meter works within specifications and if needed tuning the meter.
If a multimeter expires the calibration date, it might or might not measure at the promised accuracy anymore. Meter will still work nicely, but there is possibility that the measurement results would be slightly off from promised accuracy.
Here a video that describes basics of calibration
Here is an instructional video on calibrating multimeters:
How to Calibrate Inexpensive Multimeters
I have done calibration to my multimeters. In most cases I have used an accurate calibration signal source to make sure that the that mu meters are still in calibration. I have tried to avoid touching the calibration settings inside multi-meters, unless the following needs are met: there has been a significant error, I have has access to a good calibrated signal source or meter I can compare the device being calibrated against and calibration instructions available for the multimeter I have (I have the instructions for some meters I have).
In March 2020 I participated to a multimeter calibration event at the local HAM club. They had a very nice very accurate Fluke calibration tool for people to use at the event (owned by another HAM club) that could generate very accurate test signals to check all the different multimeter measurement ranges (DC voltage and current, AC current and voltage at different frequencies, resistance, etc..). Both of my UT-81B and my working Mastech MS8209 were proven to measure as accurately as their documentation promised.
Calibration can be made many times for a multimeter. Typically calibration is done in applications where guaranteed accuracy is needed once per year. You can make calibration check how accurate it is as often and as many times as wanted. Iy does not wear out meter more than normal measuring.
If adjustment is needed, meter needs to be opened and trimmers adjusted, which causes some wear to multimeter.