blast off to yesterday

1, 2, 3, blast off for yesterday, tomorrow!

Two men walk down several level flights of steps. The only sounds are the clanks of their shoes on the steps, and the distant plop of water on concrete. They rounded the final landing and the shorter of the two palms a pad. There was a beep, metallic click, and a thud as the door slides back.
The second man asked, “Is it true? You developed time travel?”
The first said, “Yes, but it only works in one direction.”
“Backwards, right?”
“Correct. Forward through time is impossible. Once you go backward there is no chance to come back. We have only sent people back a few hours.”
“Is there a limit to the distance a person can travel back?”
“No, at least we don’t think there is a limit. Who would want to go so far back that they would not know anyone?”
“Do you have any security measures in place to prevent someone from doing anything malicious?”
“We do extensive background checks and psychological tests to determine the stability of travelers.”
“What would happen if someone came back and killed their parents or their selves?”
“I would rather not know. The person might just die, or there is the slight possibility that the whole universe could implode on itself. My personal favorite theory deals with the sudden appearance of cupcakes and world war. Don’t ask me how, but the math makes it all make sense on paper.”