[qubex]

That's how I understand it... the monotonic rate of decay of pulsars' rotation rates allows one to backtrack to when the map was made. The question, therefore, is whether any phenomena can introduce discontinuities in those rates of decay in a manner so unpredictable as to jeopardise the time-keeping properties of the map.

[fapjacks]

Sure, it's a great question. And I don't know for sure, but I would imagine a powerful enough gravitational force, or certainly a collision with a massive body (e.g. another star) would completely change this value. But for that to have happened to enough of the 14 original pulsars so as to make the map legitimately completely useless... I think it would be an amazing, spectacular and infinitesimally-unlikely event.