I spent two weeks rebuilding the same equity backtest twice — once on Financial Modeling Prep, once on Sharadar's SF1 fundamentals via Nasdaq Data Link — and the two versions disagreed by a margin large enough to flip a "promising" strategy into a losing one. That gap is the whole story of this comparison. Both services hand you company financials over an API. Both will let you compute a price-to-earnings ratio for Apple in 2014. But only one of them reliably tells you the P/E that an investor could have known on a given day in 2014, and that distinction is the difference between a backtest that means something and a backtest that quietly lies to you. I went into this expecting the comparison to be about endpoints and pricing. It turned out to be about a single concept — point-in-time correctness — that most people discover only after they have wasted months chasing a backtested edge that never existed. This is a developer's comparison, not a trader's.…