On Quora : How can historians know the dates of things so specifically?

Most Quora questions are answered on Assorted Retorts, as per the format quora question in English ... but here I am putting it in content of philology category instead.

Q

How can historians know the dates of things so specifically?
https://www.quora.com/How-can-historians-know-the-dates-of-things-so-specifically/answer/Hans-Georg-Lundahl-1

Hans-Georg Lundahl

none/ apprx Masters Latin & Greek, Lund University

Answered 15m ago

For dates within the last millennium and some more, the events happened within the time that dates in Anno Domini have been in use.

So, an event happens, like a king acceeds to power, like Alfred of Wessex, it is noted along with death of his predecessor, Aethelred I, as one event within the “year of the Lord eight hundred and seventy one” or “dccclxxj”.

In other words, the people at the time did the job for the historians.

For events earlier than that, we have Roman “ab urbe condita” dating and also Byzantine version of “anno Mundi” dating.

Both overlap with use of Anno Domini, and both have used (since 46 BC) the Julian calendar, the same we use for AD years. Therefore, the translation is easy.

The first 708 years after founding of Rome were in other calendars, which makes things a bit trickier, and in certain cases when no AUC date is available, one has to do some adding and subtracting, and that can be tricky.

One man who did some work on it was Eratosthenes:

THE CHRONOLOGY OF ERATOSTHENES
By Darrell Wolcott
http://www.ancientwalesstudies.org/id143.html

Note, one of the things making it tricky even so is, some other old learned men had given other chronologies for fall of Troy (as is the starting point of Eratosthenes’ Chronographes).

Sometimes one has much less than that to go on, for instance with Ancient Egypt.

Here is a citation from Darrell Wolcott's paper, as bonus:

The 3rd-century BC Greek geographer and mathematician, Eratosthenes, used a diastematic system of dating events which proceeds by intervals between important events. The following chart shows his estimated dates of ancient events[1]:

The fall of Troy 1184 BC

interval of 80 years

The return of the Heraclidae 1104

interval of 60 years

The settlement of Ionia 1044

interval of 159 years

The regency of Lycurgus 885

interval of 108 years

The year before the 1st Olympiad 777

The First Olympiad 776

Other sources, including the list of Olympic game winners, tend to confirm this dating of the 1st Olympiad. We accept this date as well as all later dates proposed by Eratosthenes.

It can obviously be noted that Eratosthenes did not mark out "1184 BC" or other BC years here given, only the intervals of so and so many years.

It can equally be added that since 1580's we use Gregorian, no longer strictly Julian, calendar for AD years, but the calendars are very similar. For the time when AUC and Byzantine AM years were competing with AD, Gregorian was not yet in use.

Hans Georg Lundahl
Paris
Dedication of Our Lady at the Snow
5.VIII.2021

Romae, in Exquiliis, Dedicatio Basilicae sanctae Mariae ad Nives.