As explained in a comment by the original poster, the assumption is that at some time in the future there will be a break in chronology because people somehow regressed to the stone age or some other reason, and when they become civilized again they will have to search for a method to link their Future Modern Chronology with ours. For example, let's say that in 2117 an event happens which destroys human civilization, and that when people become civilized again they have a vague notion that they live somewhere around 2420 plus or minus 10 or 20 years.
Enter dendrochronology. Dendrochronology works by establishing a correspondence between the growth rings of trees. Today, in 2017, we have an unbroken chain of tree growth rings going back to the 5th or 6th millennium before the common era using oak trees (and possibly to even deeper time using a combination of species).
This does not help with precisely dating historical events, of course; but it would be very helpful for our confused descendants. All they have to do is to find a way to link their dendrochronology with ours; if we allow them the chance of finding some our dendrochronological records then bridging a gap of a few centuries will be easy.
But what if they don't find our dendrochronological records?
Enter astronomy. For example, in 1999 Romania put out very large number of 2000 lei polymer banknotes celebrating the solar eclipse of 11 august 1999. They looked like this:
Those are practically indestructible and there were very very many of them -- it was a very small denomination, 2000 Romanian lei of 1999 being about 0.05 Euros of 2017. The confused chronologists of the future need only to find one of those, and then they can easily compute when the eclipse took place. Note that the note helpfully says "11 August 1999" and "eclipsa totala" which, even if the Romanian language will be forgotten, is easy to translate as "total eclipse".
And even if they don't find one of those numerous and resilient notes they will certainly find some of our astronomical records.
Some our records must survive, unless the gap is very large. Ours is a highly literate society, leaving behind an enourmous quantity of chronological records. If the gap between our chronology and the Future Modern Confused Chronology is of only a few centuries they will have little trouble linking the two.
If the gap extends over several millenia so that the remains of our civilization are reduced to a few sherds of pottery, then they won't be able to link the chronologies very precisely. They will find themselves in the same situation as we are today with respect to historical events in the deep antiquity, where the difference between the "long chronology" (which puts the start of the reign of king Hammurabi of Babylon in 1848 BCE) and the "short chronology" (which puts the same historical event in 1728 BCE) is of about 120 years. (For the curious, the difference is due to us having only one astronomical record from that period, the Venus tablet, and there are four possible dates for the observations recorded by those ancient astronomers.)
For a gap of many millennia all bets are off. Our chronology is uncertain at the level of centuries for events in the first half of the 2nd millennium BCE; for even older events the uncertainty is even greater; and for events older than the 4th millennium BCE we don't even attempt to establish a chronology -- we say that they happened in pre-history and consider ourselves lucky if we can date them plus or minus one or ten or a hundred thousand years.