You have things like "in the Xth year of the reign of King Y", where we can easily relate multiple entries with different values for X, but don't actually know which CE years they correspond to. Even weirder is the Roman habit of recording "the year of the consulship of X and Y", which doesn't even allow you to relate any two different years at all without a reference table (which we don't have completely). And no, "years from the foundong of the city" wasn't a thing.
There are references to the Islamic and Japanese calendar systems, but always next to the CE equivalent.
Data entry is fortunately being done by modern people, so the translation to CE/BCE is usually baked in, and all you need to support is every possible way somebody could say "the early half of" and "5th millenium B.C. to mid 1914"
https://en.wikipedia.org/wiki/Varronian_chronology
> The Varronian chronology was adopted by the Roman state during the first century BC and gave rise to the traditional years ab urbe condita ("from the founding of the city"); most especially, those dates were used in monumental Augustan-era inscriptions, the fasti Capitolini and the fasti Triumphales.[40]
