SOLVED. YA Science Fiction: Boy bullied by twin ages an extra year in magic shed. [s]

[Nell]

Read this years and years ago, maybe late 90’s? Could be a short story.
My memory of the plot is that a nerdy boy who is bullied by his brother (twin?) finds out a shed in the backyard slows time. He decides to go into the shed for a year, bringing supplies to survive. Spends it exercising, studying, etc. Meanwhile, in the outside world, only a night is passing.
Spoiler: He emerges at the end bigger and stronger than his bully brother.

This story popped into my head the other day and I can’t find it anywhere!

[Becca]

Singularity by William Sleator?

[Nell]

Yes! That’s it, thank you!