It’s relatively easy for recent things related to big technological advances (first phone call, first man in space…); but it’s nearly impossible for really old thing, because while you can find out one really old thing existed at one time, it’s always merely the oldest known occurence. An earlier one might’ve not left surviving traces, or it’s traces might’ve not been found yet…

Big Bang, T=0.

April 16th, 1178 B.C. : the day Odysseus returns in the Odyssey.

https://www.universetoday.com/15253/homers-odyssey-may-chronicle-ancient-eclipse/.

Pretty neat

There is no definite date, but I do love the circa 1750 BCE “oldest customer complaint,” so please forgive me.

Now, when you had come, you spoke saying thus: ‘I will give good ingots to Gimil-Sin’; this you said to me when you had come, but you have not done it. You have offered bad ingots to my messenger, saying ‘If you will take it, take it; if you will not take it, go away.’ Who am I that you are treating me in this manner – treating me with such contempt? and that between gentlemen such as we are.

I have written to you to receive my money, but you have neglected [to return] it. Repeatedly you have made them [messengers] return to me empty-handed through foreign country. Who is there amongst the Dilmun traders who has acted against me in this way? You have treated my messenger with contempt.

And further with regard to the silver that you have taken with you from my house you make this discussion. And on your behalf I gave 18 talents of copper to the palace, and Sumi-abum also gave 18 talents of copper, apart from the fact that we issued the sealed document to the temple of Samas. With regard to that copper, as you have treated me, you have held back my money in a foreign territory, although you are obligated to hand it over to me intact.

You will learn that here in Ur I will not accept from you copper that is not good. In my house, I will choose and take the ingots one by one. Because you have treated me with contempt, I shall exercise against you my right of selecting the copper.

Skimming some material online, it looks like the best mechanism to get day-level dating for very old historical times are going to be celestial events, like eclipses, because we can run motions of those bodies backwards to compute precisely when the event occurred.

I searched for “first recorded eclipse”:

https://www.livescience.com/59686-first-records-solar-eclipses.html

The first recorded notation referencing an eclipse dates to about 5,000 years ago, according to NASA. Spiral petroglyphs carved on three ancient stone monuments in Ireland at Loughcrew in County Meath, depict alignments of the sun, moon and horizon, and likely represent a solar eclipse that occurred Nov. 30, 3340 B.C., NASA reported.

That isn’t a first (well, other than in being the first known recorded eclipse to us), but my bet is that it’ll be some event on the same day or within a specified number of days of an eclipse or similar.

So that probably places an outer bound on when such an event would have been known to have occurred, unless there’s some other form of celestial event recorded way, way back when.

EDIT: Though it sounds like there is some controversy as to whether that is in fact what is being depicted.

https://www.atlasobscura.com/articles/oldest-eclipse-art-loughcrew-ireland

EDIT2: and also according to the article, our accuracy in running those back that far starts to fall off:

Perhaps the biggest hole in Griffin’s theory is the date of the ancient eclipse that coincided, more or less, with the tomb’s construction. Earth’s rate of rotation fluctuates just enough over time to make calculating the path of totality for prehistoric eclipses imprecise. In fact, even programs designed to make those calculations can only do so reliably about as far back as the eighth century B.C. Steele says.

“We can’t just calculate back to 3000 B.C. and say that such-and-such an eclipse was visible in a certain place,” he adds. “The 3340 B.C. eclipse might not have been visible in Ireland at all.”

My first thought was the code of Hammurabi, as the first ever set of laws. But it turns out we know of several sets of laws from before then.

Then I was thinking maybe the Pope, given that it should be well chronicled. But it turns out Peter was the first Pope (who knew?), so we’re relying on Biblical timings which aren’t exact.

So now I’m going to say “something Chinese”. China’s recorded history goes back a lot further than Europe’s, but I don’t really know much about it to say anything more useful than that. But did you know that writing predates the iron age in China, unlike most places? (Usually, the invention of writing changes it from the iron age “prehistory” into the written “history”)