This tale of a time-travelling schoolgirl is hard to place between children's or young adult.
Between the ages of sixteen and seventeen, someone who has a gene for time travel will take an uncontrolled journey into the past. Here they will be noticeable as not fitting in and may be at risk of diseases. Charlotte was born on the correct day to fit the family secrets and has been prepared with language classes, fencing and so on for her first journey. But while she gets dizzy spells, her cousin Gwyneth is the one who travels unexpectedly to the past.
Gwyneth spends the rest of the book comparing notes with her school pal, asking questions that are not answered of adults who are in a secret society, finding out that some of her blood is needed to be added to a time machine, and taking just a couple of short trips to the past. There adults who speak to her don't believe her and she is occasionally in danger but mostly just bossed around by a young man called Gideon who has been travelling for some years.
She learns that a young couple in love, Paul and Lucy, vanished some years ago and stole the time machine, so a second has had to be put into use. There are various interpretations of their actions but they are good at hiding.
And we see various esoteric writings on the subject of the zodiac, gemstones, bloodlines and so on.
Mainly this book spends a lot of time setting up for a series to come, it feels that way anyhow. I would prefer a first book to have good strong action and characters, involve us from the start and get us hooked on wanting to read more. But this book is all about the set up and not the action. Gwyneth is an interesting girl of modern day, not above disobeying orders but mainly acquiescing to her mother's wishes. If you intend to read more of the series and see what happens to the time travellers, you will need to read the first one as it is very complicated. There is one violent scene but no bad language or heavy romance.