Do you have to recalibrate the EM sensors over time?

No.

The temperature element in Samsara EM-family sensors is a thermistor. Thermistors are temperature sensitive elements with high sensitivity and accuracy, excellent repeatability, and resistance to drifting over time.  Therefore, they do not need to be re-calibrated once deployed.  

The thermistor elements are factory calibrated for the life of the sensor and will not drift or lose calibration unless exposed to extreme conditions outside the operating range specified on device datasheets. 

Datasheets can be found at https://www.samsara.com/docs