Teenage girl (with red hair?) uses brass carnival ring to travel through time, possibly with teenage boy. Coney Island may be the carnival setting. I have an uncertain feeling that bicycles were involved somehow in the time travel, but I could be conflating two books.
Stump the Bookseller
New format, same book stumper solving.
Possibly “The Best-Kept Secret” by Emily Rodda. A mysterious carousel sets up in town and the tent flap only opens for certain people. The carousel lets the riders go seven years into the future for one hour, though no one can see them.
Maybe Ride a Wild Horse, by Ruth Christoffer Carlsen?
I found it! It wasn’t either of those, but they put me on the right track via some Goodreads lists…it’s “Back to Before” by Jan Slepian. The bike thing and the brass ring thing were right, but apparently there wasn’t actually anything about a carnival! Thanks for your help!