It's a New Year, by some standards. Because of that, I was doing some research on the Gregorian calendar and its foundations, which are uncomfortably shoddy. Our current measure, which is based loosely on the birth of Christ, is of pretty hazy certainty. To quote the Wikipedia page, "was also 525 years 'since the incarnation of our Lord Jesus Christ'. How he arrived at that number is unknown."

Alongside this, there is the uncertainty regarding the precise year of Jesus' birth, the switch from Julian to Gregorian calendar, and a few other factors which make the whole situation a little bit unpleasant to consider, personally.

My question is this: what was the first date, whether of a significant historical event or a minor one, that we know to be precise? That is, according to our current calendar. If some historians have figured this all out and made sure that the dates we reference in the distant past are, in fact, accurate, how did they do that?