One can only recommend a look at the eonic effect/model to see through this confusion over slavery.
We must notice that the two dates hover over the ‘modern transition’, beginning and end, the second near the divide. That the american republic should compromise with slavery at its start is tragic, but also, looking backward we see that the compromise, taken dialectically, provokes its own contradiction and undoing, in the crypto-teleological movement toward abolition. We can well judge that the abolitionist tide is a part of the modern transition and its compromises on the way themselves provoke the opposite outcome.
That would seem too speculative but in fact the larger reflection on history shown by the eonic model suggests that in a great irony the onset of modern slavery ignited its own abolition.
Academic historians, conservatives, and Trotskyite socialists rightly reject The New York Times’ reframing of the past.