Here:

unsigned long car_time
Time (in seconds), as measured by the car. The 0 value of this can be whatever the car module desires.
It is acceptable to merely initialize this to zero and increment it by 1 for every ticker1() received.
Alternatively, a more accurate method is to read the real time clock from the vehicle CAN bus (if available) and update from there.

I think the Tesla Roadster is the only vehicle actually updating it from the CAN bus. There was some (fairly recent) discussion about getting this from the GPS, and Tom did the foundation work (utils.{h,c} JDEpoch, JdToYMD, JdFromYMD, and datestring_to_timestamp in net.c), but we haven't completed the changes to the core net.c (+CCLK handler) to do this yet. I attach the mailing list eMail showing how this will be done.

Regards, Mark.