I'm sure in series 1 end of the earth episode the dr says its 5 billion years from now, end of the earth

however in the current episode series 4 when they are looking at a published book from agatha christie, it was published in the year 5 billion and he goes on to say people still publish her books even now

It might have been published off Earth

yeah the page read:

"Facsimile of the 1957 Earth Edition (Sol 3)

Published in the year 5,000,000,000

'Death in the Clouds' first published in Great Britain in 1935."

kinda suggests it was published on a different planet?