This book is about a boy who has an annoying little sister, and when he goes for a walk in the woods, this old man (who turns out to be him from the future) gives him a broken down calculator that helps him travel through time. That same night, his sister has a gigantic accident that puts her in a coma, and so he tries to go back in time and undo it.

  • 2
    Possibly a dupe of scifi.stackexchange.com/questions/96731/…
    – Valorum
    Commented Feb 16, 2021 at 20:08
  • I have a doubt, is asking about just books allowed? because asking books related to movies might be good idea, isn't there a SE for books?
    – user135035
    Commented Feb 17, 2021 at 12:57
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    @mTvare - books that are science fiction / fantasy have always been and will continue to be on-topic here, regardless of Literature or Movies & TV's existence
    – Mithical
    Commented Feb 17, 2021 at 13:20

1 Answer 1


This sounds exactly like The Power of Un by Nancy Etchemendy.

Description from Goodreads:

Until a week ago, Gib Finney was just a regular guy, shooting spitballs and messing up his science experiments. But when he finds a mysterious device called the Unner, everything changes. Gib discovers that the Unner has the power to erase any and all mistakes. At first, Gib thinks this is better than winning the lottery: he'll never flunk another test or strike out again!
But after a terrible accident, Gib must decide which events of the previous day he must undo to stop the disaster from happening the second and third time around. He soon learns that some things are more worthy of "unning" than others, and some things can't be changed at all.

Gib's little sister gets hit by a truck chasing after a dog, and winds up in a coma. Gib and his friend Ash use the Unner to try to prevent it from happening, and Gib winds up getting hit by the truck instead. At the end, it is revealed that the old man who gave Gib the the Unner to him in the first place (yes, it happened in a forest) was him from the future, including Gib discovering a button that he will later use to make the Unner.

There's a path through the woods, and I started to run along it, still kicking everything I came across.
Whoever it was stood in the deep shadow of a tree. The sun was about to set, and the woods felt too dark for comfort. The figure wore a long, shapeless garment—maybe a trench coat, maybe some kind of robe.
The old man loomed over me. He smelled strange—like hot metal or lightning. I thought I saw smoke rising from his rumpled clothes and a wild halo of silvery hair that stood out from his head. He had an object I couldn't identify in his raised hand.
The Power of Un, chapter 1: "Stranger in the Woods"

The old man held the box up so I could see it better. It was about the size of a paperback book and looked a little like an overgrown calculator or an electric game, but not quite like either one.
The Power of Un, chapter 2: "The Gift"

"I love you, so I'm not going to lie to you. Her head is injured. She's got brain damage, and she's in a coma. Do you know what that means?"
I nodded. "She's asleep, but she can't wake up?" My voice came out thin and tight as a wire.
Mom nodded.
The Power of Un, chapter 6: "The Big Red Button"

The doctor said I'll probably have a bad limp for the rest of my life, which pretty much answers the main question about the old guy in the woods—the stranger who knew way more than he possibly could have about my private life and who walked with a very bad limp.
The Power of Un, chapter 13: "My Thoughts About the Universe"

Found by recognizing the book.

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