So stamped on the bottom of my Nautilus it says "do not use as a booster after december 2015" and under that it says "do not use as a backless booster after december 2018" Do I really get 9 years out of it as a backless booster? (I'm using it in harness mode, and will be for a while, as its my 3yr old's seat, but I was just surprised to see that!!)