Jc3s5h added a comment. Old dates might have been determined from historical records, or might have been determined by astronomical calculations (date of an eclipse, for example). Morrison and Stephenson provide an equation showing the difference between counting actual days by sunrises or sunsets, compared to using the equations of motion of the solar system in "Historical Values of the Earth's Clock Error ΔT and the Calculation of Eclipses" in the //Journal for the History of Astronomy// v. 35 (2004) p. 332. In around 3800 BCE the difference would have been one full day. So I suggest there is no need to check dates earlier than 3800 BC.
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