Time Travelers

Time Travelers are beings who, by either scientific breakthrough, nepotistic relationships to mad scientist uncles, or sheer luck, are able to jump to different periods of time. Sometimes this is done at will, and can thus be replicated as many times as necessary. Other incidents of time travel have unique causation, perhaps by wormhole or spacial crack, and cannot be replicated.

Time Travelers as a Source of Knowledge
Time Travelers are not seen as a source of reputable knowledge, because, upon the request for proof, they often insist that you "take their word for it."