SYSTEM_TIME MAVLink Message - GPS Time

Using dronekit, I set up a message listener for SYSTEM_TIME in order to get the time since unix epoch from the GPS module. In SITL I’m receiving the SYSTEM_TIME Mavlink messages but using the firmware for PX4 Standard Plane AETR, we are no longer receiving the gps time. I need true time, not since boot up, so how do I enable the SYSTEM_TIME message to be sent to my dronekit app or how to get true gps time?

There isn’t a config.txt file on my SD card…Why?