Every GPS has to be calibrated on startup, they have to be given a date and an initial position to find out where exactly they are. From that point on, given that you do not travel too far and not have the unit turned off for too long, the GPS will recalibrate itself after that.

GPS' where also a major issue back when Y2K struck as everybody was worried that the internal clocks of GPS navigated weapons would not be Y2K compliant and due to modified navigation navigate somewhere else than their wanted target.

GPS has its own timecode, much like the Unix timestamp, which "rolled over" last year, if I remember right. GPS units track their internal date and time from the initial date and time and the GPS timecode.

So actually, since Y2K, GPS units had to survive two roll-overs, but nobody really noticed I guess.

Till