You could ad an external RTC module to the Raspberry. That will be good enough if you sync it in the home wifi before heading out.
Or get a USB GPS module and sync the time from there.
Here are instructions how to use GPS modules to update the internal time of the raspberry.
Easier version just to include in the NTP config:
https://wiki.ubuntuusers.de/ntpd/ntp_mit_externen_lokalen_Zeitquellen/
73 Joe