[aprssig] GPS Epoch Rollover Findings

[KF4LVZ]

Garmin GPS 12XL shows the correct date and time and location is accurate.
Garmin GPSmap 196 shows the correct date and time and location is
accurate (and matches the 12XL).
Garmin Nuvi 3xx series (I forgot what it was) shows correct date and
time and location is accurate (matches the above two)
Garmin eTrex Legend shows 1999 but the time is correct and the location
is accurate.  It is a bit slow to start at times, it appears to need to
download the almanac at each start.