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.