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I read this book in the mid 80's, and what I remember of it, is that time travel has been discovered (invented if you will), but it only works one way. It's possible to send poeple back in time, but it's impossible to go forward. That which has been sent back can't even be brought to its original time again.
This would seem fairly useless, but for one thing, namely imprisonment. The main character is, as far as I can remember, the leader of a group of prisoners. They regularly search they're surroundings for time junk1, which is what they call stuff that was sent back during early stages of testing the time travel. I also distinctly fossils of trilobites being mentioned.
One day a man arrives on The Anvil1, which is what they call the device (platform, scaffold, or whatever) where new prisoners and supplies appear. He claims to be a prisoner, but for some reason the main character gets suspicious of this new man, and starts following him. And in the end it is revealed that this man is an agent of the government, and that they have discovered a way to go forward in time, and that it has been decided that they have all been pardoned, and that they are free to return home. However, the main character decides to stay and he is employed as a guide to time tourists.
1Time junk and anvil are direct translations from my native language to English, but it's perfectly possible that those are not the names used in an official English version.