The most practical solution is probably to introduce a global time beacon, like a satellite broadcasting it's own time, or everyone counting the flashes of some known pulsar. Then you have one time reference everyone agrees on, it just ticks at different speeds depending on how fast you are moving.
Luckily so far the practical propulsion concepts we have max out around 10% the speed of light for reasonable amounts of propellant, so we have a couple decades left to figure this out.
