Moondial, by Helen Cresswell (1987), and since it's an old chestnut of the genre at this point, I am freely going to throw out spoiler after spoiler.
Minty (short for Araminta) is a modern girl who finds herself stuck staying with her Aunt Mary, one of the staff at an old stately home, when her mother is injured in a car accident. As Minty explores the gardens, she finds the Moondial, whose magic slips her back in time. Sometimes she is one century back, in the summer sun where she meets Tom, a scrawny, consumptive kitchen boy who wants desperately to be reunited with his little sister still in London, and sometimes Minty is back two centuries, where she meets Sarah, a mysteriously lonely little girl who comes out only at night, and who is cruelly persecuted by the other children when they catch her.....(she has a dark, vivid birthmark on her face, and has been told, among other cruel things, that if she looks in a mirror the devil will take her soul...) This is all very atmospheric and kind of spooky and fascinating, and Minty gets to know Tom a bit, and is sorry for him, and she and Tom both see Sarah, but not so much to talk too because of her circumstances (being screamed at for being a devil child by the local kids of her time, or being dragged indoors by her fiercely mean governess....)
My recent reading of this one was my second time, and at this point in my re-reading, about half-way through the book, I was wondering why I hadn't liked it the first time I read it, because it had all the things that made for good mysterious timeslip fun--vivid descriptions, emotional resonance, mysteries about how the fates of the three kids were entwined, etc. Then a nasty old ghost hunter woman comes to stay at Aunt Mary's. As well as being unpleasant in the worst sort of grown-up way she is possibly truly evil, and she is inexplicably identical to the Sarah's evil caretaker back in the past (which remains annoyingly unexplained--are they the same evil woman or not?) and then Tom and Minty try to save Sarah, and do, sort of, by convincing her to look in a mirror, but then Tom and Sarah seem to end up dying, with no other resolution to their stories other than that they are "free" (which then makes the reader wonder to what extent they were stuck ghosts...) and the book ends.
So the plot just does a "whatever" throwing up of its hands and everyone is unstuck and Minty's mother recovers and it is all sort of an anticlimax.
The Moondial itself is a lovely fictional creation, and poor lonely Sarah is as haunting a character as one can hope to find, and I have no objections to Minty at all, but I think you have to be nine or ten when you read this one the first time because then the atmosphere and mystery will be enough to make up for the lack of crisp resolution...and the book will haunt you forever and you will love it always. Kirkus gave it a star back in 1987, and I don't blame them, because there are strengths to the book. It just let me down at the end.
Short answer--if you see it a library book sale for 25 cents, definitely worth it!