Observer A says that event X happened before Y.
Observer B says that event Y happened before X.
And they're both right from their POV.
There's no preferred frame of reference, meaning that any frame of reference could work, but you can derive one from the cosmic microwave background if you don't like picking something too arbitrary.
Im not sure pulsars are actually that precise and predictable to not require regular adjustments to equate local times, but it would provide a standard anybody could utilize as long as they could keep an eye on it.
Of course it would break down if someone broke light speed, either with wormholes or some unknown technology, but as long as you are below lightspeed then there shouldn't be any problems.
Due to their small size, pulsars are relatively weak radio sources. Therefore, the largest radio telescopes in the world are usually needed to observe them. As we have seen, pulsars emit their largest intensity at low radio frequencies around 400 MHz. In particular at such frequencies, however, the pulses suffer from propagation effects when they travel to Earth through the interstellar medium. [0]
[0http://www.jb.man.ac.uk/distance/frontiers/pulsars/section4....
It's even useful as a consistency model for some distributed systems.