Men need to place events in time, need to give events a date; but the different cultures and peoples date based on several events, interesting to them, but irrelevant for other people.
Man, a being with memory, moves and locates on two essential coordinates: space and time. To be effective, that memory has to place any fact, any event, any memory at one point in space and at one moment in time. Help comes from geography ("ge" in Greek means land) or the knowledge of the physical environment and from chronology ("chronos" in Greek means time) or the historical dating.
An "age" (era) is a time period whose beginning coincides with a conventional event from which on we start counting years. As it is well known, we arrange facts and we date the facts from the birth of Christ, placing them before or after. That year is matched with 753, the year of the founding of Rome. But logically, this was neither always the case nor it was like this everywhere.
In the beginning of ancient Greece, each year was recognized by the name of the most important magistrate, Archon, also known as "eponym" (Greek "epi" means "on" and "onoma" means name) . In the fourth century B.C. the historian Timaeus Tauromenium started dating with the first Olympiad, which was held in 776 B.C. Considering that the Olympics were held every four years, the third year of the second Olympiad corresponds to 766 B.C., and so on.
In Rome, years were also given the name of the two most important annual magistrates, consuls, saying for example, "being consuls … and ….". Much later, historians dated from the founding of Rome, which was set in the fourth year of the sixth Olympiad, and used the statement "ab urbe conditia = since the founding of the city." Later on, in the sixth century, that year was made to coincide with the 753 B.C.
The Christian era begins in the year 753 since the founding of Rome, i.e., in the first year of the Olympiad 195. This is year 1 of Christ, because it was set by the monk Dionysius Exiguus in the sixth century. But we have to know that Dionysius erred and Jesus Christ would have been born probably about four years earlier.
Of course, no one has now any interest in correcting this error, an ancient error, whose remedy would only complicate everything and would create many problems.
But there have been many other eras according to peoples and cultures: the "Spanish era" started the year 38 B.C. , without knowing the reason for it; the “Byzantine era” that set the creation of the world in 5508 B.C. and was used in Russia until 1700; the “era of Abraham” which begins on October 1, 2016 B.C.; the Hijra (al-Hiyra = هجرة) or “Muslim era” that begins on July 16th, 622, when Muhammad leaves Mecca to Medina; the “era of the French Revolution” which begins on the 22nd of September 1792. And there are many more, in other parts of the world.
The computation of time and age dating, that might seem so simple, are in fact quite complicated, especially when someone is determined to raise as a great moment for himself an event that for other men in other places do not have any meaning.