A computation of time, which ascending from a given point determines its earliest epoch by artificial combinations, may seem unfit for and unworthy of being used in chronology. But for practical purposes nothing more is requisite, than that the point it begins at be fixed relatively: the first year even of our own common era is notoriously misplaced: only such chronological determinateness must not be mistaken for historical certainty. The dignity of Rome purges its era from the blot of having owed its origin to fraud.

History requires more than one era; Asia a different one from Europe: such eras as reckon backward, or are necessarily dependent on a supposition ascertained to be utterly wrong, are positively bad: different eras are suited to different times; thus the Spanish from the battle of Actium was appropriate so long as the Western empire lasted: afterward it ought to have given way to the general Christian era much sooner than it did; as that of Nabonassar was very reasonably made to yield to the Seleucidian. The greater or less value of an era for practical purposes depends on three qualities: that it begin early enough to comprehend the period of such dates as are really historical, within its sphere in its forward course; that this sphere without straining include the history of the most important nations which come within it; and that the reason which entitles the era to preference, remain long unaltered.