That is if they go for interstellar navigation. It's totally possible that you set your ship trajectory, fire your engine, and once you leave the kerbol System you ship disapear from the game. at that point, you unlock a new "stage" that is independant from the kerbol system. I say this because i wonder if the game can actually keep track of several systems at the same time. with that model, if you are playing in anothger star system, the others are paused, and each system would have their own time, or that the other systems are on pause and that the game will ajust itself for the time you spend in another system when you return to kerbol for exemple. that way it is not tracking several star systems in real time.