[kragen]

I think hundreds of years ago the norm was to count years from the beginning of the era, typically the rule of the current emperor, e.g., 文政five年. (In that system it's currently 令和3年.) Of course that varied by locality; in much of Europe, for example, the norm was to use Roman numerals counted from a year that Dionysius Exiguus had miscalculated as the year of Jesus's birth, e.g., MDCCCXXII.