Planetary reference systems and time
Look at J2000 Solar System Barycentric (spatial reference) and Barycentric Dynamical Time (TDB).  Together they are good space and time coordinate system.  NASA/JPL has some good info and data on these.

In terms of time, TDB is rescaled so that from the Earth, it appears to be close to the same as TT (roughly ~UTC). The rescaling is due to the fact we are in a gravitational well, as well as moving at 30 km/s with respect to the solar system barycentre, due to relativity. The original unscaled is called Barycentric Coordinate Time (TCB) and differs by ~0.5seconds / year.