Because it's impossible. There's no such thing as a universal time; it's all relative to wherever you are at that moment. Earth is small enough that relativis

Because it's impossible. There's no such thing as a universal time; it's all relative to wherever you are at that moment. Earth is small enough that relativistic effects between any two points are insignificant, but the Moon is far enough away that it'll be noticeable, though not that much. Anything farther, however, and it all breaks down. Within the system, it'll probably be "close enough" to have an offset between different places. If we expand to different star systems, though, it'll be impossible because they're moving in different directions.