My guess was based on the fact that Garmin are based in Olathe, KS, USA which is currently 6 hours behind us.
So I wondered if in the absence of Time Zone information, it defaulted to that time zone.
Hmm, I just tried renaming-away that file.
The device booted up and got the time zone right, but the Daylight Saving wrong.
It thinks it's an hour later that it is.
I think there's another file which it uses to maintain the last position / time to assist with getting a quicker fix.
I can't remember what file that is, but I expect a full reset deletes it.
I suspect if I delete that, I may be able to repro the 6-hour time shift.