"Like a book with the date written in it", assuming they had a written language, which they didn't, this would be MUCH easier to fake than carbon dating. Hypothetical scenario:
I do some research on ancient cave paintings, learning the materials they used to write the paintings, etc. I make up a language, I go into some cave in Africa, I write a message on the wall of that cave stating some events, perhaps astronomical, that could be used to pinpoint a date, such as a rare alignment of planets, a solar eclipse, etc. And I also give some human event that is theorized to have occurred, such as a war or conflict of sometime. Now, since I made the language up, no one can understand it. So, I write, hidden somewhere, a translation from my made-up language into Greek. That may seem farfetched, but the Rosetta Stone did this very same thing to allow people to translate from Egyptian heiroglyphs, so it is believable. When the translation is complete, it refers to events that pinpoint the message to be from the year 15,000BC. Based on your defintion of "solid proof," this certainly qualifies as proof. However, if carbon dating were done it would reveal that I wrote the message last Tuesday, not in 15,000BC. So the carbon dating, which you say is so inaccurate, would in fact be the method that would conclusively disprove what you claim to be solid proof.
I'm not saying you should neglect written records because they may be inaccurate, I'm saying you should use all of the tools at your disposal rather than deem some, such as carbon dating, to be an inaccurate and therefore a useless tool. In the scenario I presented, it is one of the only tools that would have lead to the truth.
