Proving one is a time traveler to people in the past is difficult because simple, mundane explanations for one's behavior are always more plausible to them than the truth.

“There's just too many easier explanations for their behavior and knowledge than the truth, which is that they actually travel through time.”

A time traveler can establish credibility in the past by providing multiple, verifiable coordinates for famous lost artifacts or ships.

“But with multiples, especially different types of things like a shipwreck, a gold horde, and an ancient manuscript, come on. You're a time traveler. You're from a time when these things have been found.”