Relative to the modern calendar. For instance, do we know exactly when Caesar was killed? Is there anything from hundreds of years before then that we can put a precise date on?

To be clear, it's not enough to merely label the event with a date, because I understand there have been irregularities in the convention. I want to know what is the earliest significant date for which we could, in principle, count out exactly how many days ago it was from right now.

Does the answer change if we lower our precision to just the season and year?

(Edit: I feel silly for my Caesar example, as I just remembered the "ides of March". Still, has the calendar remained consistent over that time span to the point that we can figure out when it was?)