A minus sign does indeed mean the car is that much past servicing. I'm not sure if it represents miles or days for everyone, but mine was definitely counting days.

It was at -59 when I bought it new, and added one for each day that passed. Until yesterday when I had my car serviced for the first time (4,200 miles), the FSS display was up to about -280. Doing the math takes you right back to the date I bought my car.

Now it reads 7,000.