I vividly recall this book I read when I was a kid in the '70s.
I thought it was called Saturday the 14th, but I am having no luck with that title.
It's about an adolescent girl having a really bad day that culminates in her finding her brother reading her diary aloud to a friend of his.
She runs away and sits by a tree, and ends up traveling back in time to probably the late stone age, I know there's language because it's a pivotal plot point when she starts speaking their language.
I don't recall much of the middle part, I know she starts her period, which involves some ritual in this culture. I want to say she spends a year and a half there and when she comes back it's the same day, no one has noticed she was gone, and there's some kind of resolution with a conflict with a teacher -- basically, she's matured.
I cannot find any sign of this on the internet. It's rare that I recall a book so vividly and would love to get my hands on a copy for the kids in my life.