GPS satellites already control for relativistic effects. Their required accuracy is so high that they wouldn't work without doing so.
You would simply also correct for relativistic effects caused by the orbits of different planets, and boom, you have a pretty consistently defined time that is valid across the entire solar system.
The only thing I can think of involves truly relativistic speeds, which just isn't going to happen for human travel anytime soon, if ever, and certainly not within the solar system.
You would simply also correct for relativistic effects caused by the orbits of different planets, and boom, you have a pretty consistently defined time that is valid across the entire solar system.