I was still using a GT-11 gps until recent. It seems the Amaryllo GT-11 is suffering from the GPS week rollover phenomenon. Below from wikipedia.

The GPS week number rollover is a phenomenon that happens every 1,024 weeks, which is about 19.6 years. The Global Positioning System (GPS) broadcasts a date, including a week number counter that is stored in only ten binary digits, whose range is therefore 0–1,023. After 1,023, an integer overflow causes the internal value to roll over, changing to zero again. Software that is not coded to anticipate the rollover to zero may stop working or could be moved back in time by 20 or 40 years.

All recent sessions are now backdated to somewhere in 2003. Tried to reload the firmware but until now unsuccesfull. Anyone who knows a fix for this other than buying something new :-)

Use the track filter in GPSBabel to shift the recorded times up 1024 weeks. See

https://www.gpsbabel.org/htmldoc-development/filter_track.html#ex_track_move_wnro

for the basic recipe. Of course, you'll have to sweeten "-i WHATEVER_FORMAT -f WHATEVERFILE to taste. For -31, I think sbp or sbn are used, but I'm rusty.

Craig?

I wrote a simple python program that adjust the date in .sbp files. It can be used for the GT11 / GT31 which suffer from the gps week rollover problem :  https://github.com/RP6conrad/GT11_gps_week_rollover
A windows executable is on github, for Mac / Linux, you have to install python and run the python sketch.

Thnx @Jan Heynen, very usefull !