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Haven't read it since I was a child, but I recall a story involving a young man who's father built some kind of time portal that appeared as a circular rainbow. ("No end, therefore no pot of gold" was mentioned at one point.)
During the story, I believe they move thru it at least once or twice in it's early, dinosaur era incarnation, and at one point have it suffer major issues when a dinosaur steps on it, and a tube shaped volume of Dino comes thru it.
Later, it's pointed at a later (historically) time, where the protagonist befriends a young girl (and makes her some clothing, which she later takes off, points at him and puts back on, to his embarrassment, as she tries to explain where it came from) who, at the end of the story, inadvertently came back with them to the 'modern' age.