Feature request: increase robustness against clock errors by using the time/date from the GNSS itself

The raspberry currently gets its time by connecting to the internet and querying a time server (see the systemd service: systemd-timesyncd.service).

In the event of power reboot in combination with a internet connectivity failure this can (and has) caused problems as the filename used for logging uses the date derived from the system clock. When it's off, old log files can be overwritten or tagged with incorrect dates.

There are several solutions for this: (1) Instead of enabling the systemd-timesyncd service create a custom timesetting service which sets the system time when a GPS fix has been obtained. The correct date and time can be extracted from the nmea messages. Note that setting the time requires elevated privileges (sudo) (2) Don't use the system clock at all for file naming, extract the filename directly from the nmea messages and rotate every day.