In The Restaurant at the End of the Universe, Arthur, Ford, Zaphod, Trillian, and Marvin have just stolen Hotblack Desiato's stunt ship after leaving Milliways and traveled into the past 2 million years before their own time.
”Trouble is,” said Ford, ”that the one instrument in this whole ship that is giving any reading is worrying me. If it is what I think it is, and if it’s saying what I think it’s saying, then we’ve already gone too far back into the past. Maybe as much as two million years before our own time.”
As they realize that this ship is about to crash into the sun, they discover a teleportion device (their only means of escape) and leave Marvin behind to control it so they can escape despite the fact they have no idea where they might end up. Arthur and Ford end up on the Golgafrincham ship which shortly thereafter crash landed on pre-historic Earth (i.e. roughly 2 million B.C.), and Zaphod and Trillian end up back on The Heart of Gold.
”I trust you had a pleasant meal?” said Zarniwoop to Zaphod and Trillian as they rematerialized on the bridge of the starship Heart of Gold and lay panting on the floor.
As I understand from the beginning of the book, at this point the Heart of Gold had been left with Zarniwoop alone on the ship in Arthur's "own time" (in the 1980's). There doesn't really seem to be any explanation as to why the teleportion device sent Zaphod and Trillian 2 millions years into the future as far as I can tell.
So, am I misinterpreting something to the story and the gang really left The Heart of Gold 2 million years before their own time, or did the teleportion device also act as a time machine when Zaphod and Trillian stepped into it? Or perhaps did Zarniwoop have The Heart of Gold travel back in time?