I’m trying to send SYSTEM_TIME messages from the GCS (my laptop) to Ardupilot 4.0.0 (Rover) using MAVProxy.
Here is what I type into the console
I’m also connecting to MAVProxy with Mission Planner and I’d expect the time_unix_usec field to be updated according to my GCS time. It isn’t the case, as you can see here

How can I tell if the system_time module is working properly?

EDIT: ok, I realized that you must also set the BRD_RTC_TYPES parameter appropriately.

Second question: is system_time logged in DataFlash logs? If so, where?

I believe it is logged. Download the dataflash and use to convert the binary log and find the message.