[not a solution, but info worth sharing]

Hi Jonathan,
when I read your description my thoughts went back to that time my Garmin
Geko 201 (small GPS tracker) recorded a date 15 years back in time.

Digging around I discovered that GPS/NMEA carry a 10 bit "week"
information, which makes a rollover every 1024 weeks. But quoting a search
result (written in 2013):

The last rollover (and the first since GPS went live in 1980)
was 0000 22 August 1999; the next would fall in 2019, but plans are
afoot to upgrade the satellite counters to 13 bits; this will delay
the next rollover until 2173

So what you are seeing is not due to a week rollover, even though my Geko
201 showed the problem in 2010 (and others too).

Perhaps Motorola hardcoded a "starting date" that matched a week number and
counted from that point. Then a software variable has overflowed or 1024
have passed and you're back to "Oncore UT day 0".

If the delta_days is constant you might add it to your code right after
reading the date information? You might discover is it some integer number
falling within powers of 2. :-)

Paolo

PS: Garmin has issued a firmware update for the Geko 201. But I doubt I
will get a new firmware in 2019...

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