Let's say I've been able to travel in time and I am back in the past (just by a few years or even centuries). I do not have any modern gadgets or even modern clothes, just me and my 21st century mind.

I do not have enough time to prove that I am from the future by predicting future events (like a future president or an environmental catastrophe), but I did prepare for a while using the answers to this question, so that any current scientific knowledge is actually usable.

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How would I prove to other people that I am from the future?

Talking to a medieval person about Quantum Mechanics would just get me killed, since I could not prove anything.

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What's the upper limit of time from which I will no longer be able to prove my identity?

For instance, if I just traveled back a year, I don't think I'd be able to prove that I'm from a more scientifically advances society.

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Let me know whether I've been clear or not.

Thanks in advance.

I found [this question](https://worldbuilding.stackexchange.com/questions/45952/when-transported-suddenly-to-ancient-persia-how-to-quickly-show-that-i-am-from) but it is a little too specific. I would like a reference to the scientific knowledge of each "ancient" society.
Furthermore, my question is not about *surviving in the past*, but about *actively proving* to other people that I come from the 21st century.