[saintgimp]

Relativity kind of messes with our notions of time.

Any event that happens outside of our lightcone (http://en.wikipedia.org/wiki/Light_cone) cannot have any casual effect on us, even theoretically, until it enters our lightcone. Before then, the event literally doesn't exist from our point of view. Because of this, it's reasonable and valid to take the position that distant events "happen" when we're first able to see them happen. It's not the only perspective you could have, of course, but it can be a useful one and astronomical events are often discussed this way.

[Houshalter]

That's not true if faster than light travel is possible (e.g. wormholes, warpdrives, any form of time travel, etc.) And even so, it doesn't make a lot of sense to redefine time that way. It would be like a pre-electricity civilization declaring that things outside their mail-cone (how fast mail can travel) haven't actually happened yet.