All metrological instruments need to be recalibrated regular. The recalibration period depends on country regulations. In most countries portable calibrators have a recalibration period of one year.
The recalibration can be done in any for this purpose accredited laboratory.
The general principle is to compare the calibrator pulses with the reference standard pulses. Connect the P-OUT to the pulse input and GND to the ground, calculate the meter constant for the actual load and verify the RS350 accuracy with the higher accurate standard.
The RS350 can also be adjusted locally at any metrological institute.