This book was a short picture book that I read in the 1970’s. I am guessing it was published late 1960s to mid 1970s.
A girl visits her great aunt or similar relative, who is bedridden and lives in a room upstairs in a beautiful old house. When the clock strikes the old lady turns back into herself as a young girl and the two girls play together and have a great time. Then the clock strikes again and she turns back into an old lady. The girl is dressed in what children would have worn in the 1890’s or thereabouts and I remember she had long black hair with a ribbon tied in it.
Sounds like “Tom’s Midnight Garden” by Philippa Pearce. The modern character is actually a boy (Tom); he is staying with his great-aunt and uncle due to his brother’s case of the measles. The woman/girl he plays with is the owner of the apartment building.
It does have some elements in common with Tom’s Midnight Garden, but I think the one you’re looking for is Who is Victoria? by Betty Erwin. It takes place in one day, and the girl with the mysterious old-fashioned clothes definitely has long black hair.
Thanks very much for your replies. ‘Tom’s Midnight Garden’ has so many similar elements that maybe I am confusing two books and it really is this one. The name Hatty rings a bell. It definitely involved only two main characters so I don’t think it is ‘Who is Victoria’. I was convinced that the book was a large color hardcover only about 15 pages long and I can still vividly picture one of the illustrations, but now I am not so sure. I have ordered ‘Tom’s Midnight Garden’ from the library to see if the light bulb goes on. Thanks again.