The last one. Every signal the engine has ever produced was written to the chain when it was produced, with the time attached. We cannot add one now that we wish we had made, we cannot remove one that turned out badly, and we cannot nudge a confidence score to look better than it was.
So a test run here is over the complete history, including the stretches nobody would choose to show you. That does not make the other three problems disappear. You can still hunt for a flattering set of rules, and you can still pick a kind stretch of dates. What you cannot do is quietly bury the runs that went badly, and that is the one that made every other backtest in this industry unreadable.
It also means anyone can check your working. A published strategy's history is not a screenshot in a marketing page, it is a thing on a public chain that a stranger can go and read for themselves.
And because the whole record is there, you can run the same rules across one stretch after another instead of the one that suits them. A set of rules that only works in a single four month window is a set of rules that found that window, and moving between periods is the quickest way to catch that happening.