The FSS initially shows miles/kilometers but it senses if the cars hasn't been driven for a while and then goes to days till next service.
Mine was in an accident and the body shop had it for 75 days. Before the accident I had 3,400 miles on it and it showed 3,600 left till next service. When I got it back it showed 20 DAYS till next service and kept counting down each day whether driven or not till it was in minus days. It says it in the manual that it does that automatically when the car isn't driven very often. I guess they want to make sure you don't go 4 years without an oil change if it's your Sunday car.
I just rest the FSS myself. I'll get my oil changed at maybe 6,000 miles to be safe but I don't think it's that bad for your car.
I have family in Europe and they don't change oil as frequently as Americans do. Granted, they don't put as much mileage on either but they do mostly the schedule b driving (stop and go, city driving) My mom had a car for 10 years and maybe changed the oil once. Never a problem.