And here's how to do it in a global basis, so that
anyone that went back in time could save the future.
Have a computer generate a random password, in such a way that no one reads it. It shouldn't be hard with the proper equipment. Now shut it tightly and bury or hide it somewhere.
Now, whoever is going back in time only needs to ask the people that hid the password to uncover it and tell him. He comes back in time and tells the password: The past people dig out the password and voila, it matches.
Of course, if someone dug the password out, he would become the Herald of the Future. But that's a problem for the hardware engineers.
The Cow says:
the password can only be used once- which means they'll have to convince them that he's from the future before they can read the password...
True. Perhaps a large array of passwords could be used - ten thousand, maybe more. If one is proved wrong, that password is simply destroyed and cannot be used in the future.