GP-80 stop position calculation, it also shows incorrect date and time.
GPS week number "rollover" appears every 1024 weeks - ie about once every 20 years.
The GPS system calculates the date by counting the number of weeks over a period of 1,023 weeks.
At the transition to the beginning of the 1024 weeks, the GPS "weekcounter" reset to 0 weeks (Time Scale Origin).
As a result of this, the GP-80 view the incorrect date / time information.
The solution is to replace the internal GPS core display or switch to the new model GP-150.