The question comes to mind when the Enterprise engages into warp drive. The way warp drive works, as I understand it, is that depending on the amount of energy expended by the Enterprise determines a specific warp factor the Enterprise is traveling. That aside, each warp factor is a measure in multiples of light speed.
In context to Einstien's thoery of relativity, there's a sort of consensus in that as you aproach the the speed of light, time starts slowing down. Furthermore, at the point at which exceed the speed of light point time travel occurs. Does Star Trek use a different theory?