What does RF Cal means?
RF Cal stands for "Radio Frequency Calibration". For smart phones, it means "time of first power up". After assembly, some parts of the phone(e.g. magnetometer &amp; proximity sensor, radio chips and antennae)  must be calibrated before it's correctly functional. It is slightly different from "manufacture time", because hardware calibration is after the device is fully assembled. You can simply think it's "time of the end of manufacture of this device".