SOLVED!: YA sci/fi in which children time travel into grandfather's past starting with message on moving van [s]

[Raymond]

Bickering children heading to Florida vacation see a message on a moving van relating to time travel. They don't understand this, of course. Parents drop them off with grandfather. They somehow get to travel into his past to key events in his life. I read it in the early 1990's from a junior high school library.

[Emily]

The mention of the van reminded me of Shadow on the Dial by Anne Lindbergh, but I don't remember the book well enough to say whether the other details match.

[Raymond]

Thank you, Emily. That's it!