Circa AD 1800 to 1899: Not much earlier than 1890 because “Red River Valley” was written circa 1890s. And no later than 1895, since that's the date that Slim’s book was published.
Causal Loops: A bit of a causal loop when Slim’s book causes Annie and Jack to travel back in time, and when they are back in time, Annie encourages Slim to write his book. I think this falls short of an artist paradox because Slim never sees much of the book, nor do Annie and Jack tell him much about it.
From the magic tree house in Frog Creek, Pennsylvania, circa 2019 ⋙ to outside of Rattlesnake Flats, circa 1890–1895. Round trip. Note: Jack and Annie in the tree house.
“Slim, you should write your book,” said Annie.
3 English variants