I was wondering the same thing. The closest I could find to answer is this:
> The Long Now Foundation uses five-digit dates to guard against the deca-millennium bug (the “Y10K” problem) which will come into effect in about 8,000 years. As you may have noticed any reference we make to a year begins with a zero: 01977, 03012, 02000, 00521, 01215, etc.
What a useless thing. Omitted leading zero has been the norm for hundreds of years, there is no hidden assumption. This would only be a problem if after 10k you omit the leading 1.
Why limit ourselves to the year 10k “bug”? I think we need to be future proof and prepare to the year 1M bug by writing years with three leading zeroes /s
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.
