Survival Kit by Frederik Pohl — Read

Mooney stared at his napping guest in a mood of wonder and fear and delight.

Time traveler! But it was hard to doubt the pale-eyed man. He had said he was from the future and he mentioned a date that made Mooney gasp. He had said: "When you speak to me, you must know that my. Name? Is Harse. " And then he had curled up on the floor, surrounding his shiny briefcase like a mother cat around a kitten, and begun dozing alertly.