AUC 21st April 753 BCEIn this post our own James Ussher, Primate of All Ireland meets Marcus Terentius Varro, a contemporary of Caesar adsum iam forte as well as Pompey aderat (for those who never had Latin at school, it will help if you say the italicised phrases aloud to appreciate the joke). The connexion is that they were both accomplished calendarists, albeit separated by 1800 years.
Varro is sometimes know as Varro Raetinus because he came from the town now known as Rieti which is close enough to being the geographic centre of Italy. He is responsible for deciding that what we call 753 BCE was Ab Urbe Condita (from the foundation of the city) from which Romans counted their years. He was a gentleman and a scholar, who fought on the wrong side in the civil wars, was pardoned and so given time to fossick through the incomplete records listing the Roman consuls, interpolate between equivocal data points, make an informed guess about the really ragged gaps and come up with a date for when Romulus and Remus had their spat and the winner built a city. That date is today - 21st April; insofar as you can match dates then with dates now, having had the Julian and Gregorian ("gives us back our eleven days ... ye baastids" etc etc) calendar reforms in the interim.
Scientists of the less tolerant variety snicker at Ussher for having given the biblical fundamentalists a certainty about when their god started making the world. But he also was a gentleman and a scholar and knew his literature a lot better than a great many scientists know their. He also lived through a civil war, and I like him for being sensitive enough to faint while watching the execution of Charles I. His chronology required considerable expertise, multilingual skills and years of dogged research. Those who know credit him with considerable accuracy where there were data (astronomical, historical, archaeological) to cross-reference . But when he was forced to rely exclusively on the bible and its internal inconsistencies, the accumulation of error got him into the fix of pronouncing that god's second task (immediately after the Fiat Lux! business) of "separating the light from the darkness" happened in the evening before 23rd October 4004 BCE. And, in fairness, the title of Ussher's magnum opus is Annales veteris testamenti, a prima mundi origine deducti - he was only trying to sort out the annals of the old testament, which as I say were in rag order because the primary data were inconsistent. He was conscious of his own special pleading in setting the creation exactly 4000 years before the birth of our lord; and also the foundation of the temple exactly 3000 years later - it helped fulfill the prophecies.
And everybode kno that the first attempt at pegging the year of the birth of Jesus (1 AD - because Unix hadn't been invented, they weren't used to doing their counting from 0) was out by a few years and that the delightfully named Dionysius Exiguus (Dennis the Short) sorted it out so that it was consistent with Roman history in around 500 CE.