Then you've got cultures that state the year according to the reign of the current king. If that king was crowned during the same year that his predecessor passed away, then you've got the first year of the reign of King Whatsisface IV being the same year as the last one of King Whatsisface III. If crowned the following year, then those years are different, so simply adding up the years of each king's reign isn't necessarily an accurate way to gauge the year of an event. Then you figure in "regencies" and cases where the heir was on campaign or busy putting down a rival for the throne, and not able to be crowned that year, meaning that there's another year or more unaccounted for in the timeline. That makes any chronology of ancient times subject to quite a bit of variation over time, to the point where we can't definitively align the timelines of several major ancient cultures to within even a decade of each other. There's simply way too much room for error, and too many exceptions to the rules to be able to confidently state that some particular event happened in 843BC, for example. Worse, there are very few international events that tie even two of the civilizations together beyond a reasonable doubt ("Was that the SAME "King Whatsisface" mentioned in the inscription, or his great grandson with the same name").
The use of regnal chronologies is admittedly a problem. We may know that king X ruled for 10 years, followed by king Y for 5 years, but doesn't tell us when exactly. We only get relative rather than absolute dates. Since relative chronologies ("third year of King X") are the most customary way to record events, it poses a problem.
Happily, Ancient civilizations were not completely self-contained, and events in one often had implications for another, and so were recorded in both (Egypt records its invasion of Hittites, Hittites record being invaded by Egyptians). So we can reconcile cross-civilizational records and approach a more correct date.
Doesn't always work, particularly if records within a civilization are crappy or contradictory (e.g. Egyptians have multiple chronologies that don't match up with each other) and names are often ambiguous (esp. if repeated, is it in the reign of Ramses the father or Ramses the son? how many Ramseses are there exactly?). So you're right.
But this is not really as much a problem for the Roman Republic. Happily, consuls are annual and there are at least two of them, so Roman consular dating is quite "fine-grain" relative to typical regnal chronologies, with reigns of indeterminate length (only one year for consuls) or repeated names (can't tell if father or grandson? check who the other consul was).
The Ancient chronologies for Rome rely heavily on matching relative consular dating with the Greek Olympiads (which were probably the best absolute records of the time). This was done by several ancient Roman historians themselves. The major effort to pin down absolute Roman dates by matching with Olympiads was undertaken by Polybius in his
Histories. It culminated in the AVC system of the
Annales of Marcus Terentius Varro.
So for Roman dating we have three systems:
(1) By the "consular year", that is "in the year of consuls x and y". This is relative dating and relies on the
fasti consularis, a fairly complete list of consuls from 509 BCE forwards. Since Roman records are cited by consular date, this is the most "accurate" match from events to year, but it is still relative.
(2) By the "triumph year", that is, "in the year of the triumph of z". This relies on the
fasti triumphalis, a record list of triumphs held in Rome, which is more incomplete. But helps double-check matching.
(3) By "AVC" (
Ab Urbe Condita), that is, "years since the founding of the city" (pinned by Varro at 753 BCE). This is the absolute measure created by Varro, relying on Polybius and Greek Olympiads. The Varro chronology became common and official under Augustus.
Of course, none of this is perfect. Roman histories still contradict each other at times. For instance, Livy's chronicles have a different AVC (751 BCE) and his account of the early republic is very crappy, his relative consular dating wanders away from the list for years on end and he obviously makes up stuff ("this year no consuls were elected") to try to reconcile things. The chronicle of Dionysus of Halicarnassus's (a foreigner, with foreign sources) is much better and matches the
fasti much closer (although he pins AVC at 752 BCE). Alas, since modern writers tend to read & love Livy and often overlook Dionysus, Livy's mistaken dating tends to be perpetuated.
Still, the contradictions aren't as vast as all that. They may be off by a couple of years, but not "by decades" (like the Egyptians).
