The Showa Hundred Year Problem

This is one of those things that cause confusion: There are two sets of centuries, each mostly overlapping but not quite identical.

There’s the numbered centuries: e.g., the twentieth century, which ran from 1901–2000.

Then there’s centuries named by the first two digits: e.g., the 1900s which ran from 1900–1999.

I would note that decades are always indicated by the first three digits and (almost¹) never as numbered decades so when people talk about the ’20s they’re referring to 1920–1929 or 2020–2029 and not, e.g., 2021–2030.²

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1. I’m sure you can find somewhere someone talking about the first decade of the twenty-first century, although that’s likely an attempt to get around the awkwardness of not having convenient names for decades like 2000–2009 or 2010–2019.

2. Every ten years you’ll find tedious people who will insist that the decade doesn’t end in ###9. Don’t argue with them. Just walk away.

Wait until you learn about companies that use their founding year as the first year on their calendar.

But also, it is, in fact, changing. A lot of official government dates (such as drivers licenses) are no longer going to use the imperial year system.

I might be misremembering, but if I remember correctly, back in 2019 Windows' pre-installed calculator had an interesting issue with the Japanese era change in 2019 from Heisei to Reiwa, due to Emperor Akihito's abdication and the succession of his son Emperor Naruhito. I was not personally involved with this issue but I was aware of it as I was working on the Windows team at Microsoft at that time.

The announced plan was that Akihito would abdicate on April 30 and Naruhito would take the throne on May 1, so that Jan 1-Apr 30, 2019 would be Heisei 31 (平成31年) and May 1-Dec 31 would be Reiwa 1/Reiwa Gannen (令和元年).

The issue was with Windows Calculator's date calculation. If you ask it which day is 61 days after April 1, 2019, it correctly calculates June 1, 2019. The question, though, is what if you have your Windows settings set to show the Japanese era year - should it show "2019" in "June 1, 2019" as Heisei 31 or Reiwa 1?

If I remember right, the answer chosen was "it depends on what your computer's current clock time is!" In other words, if you ask Calculator while your PC's clock is set to April 2019 or earlier, it "wrongly" shows June 1, 2019 as in Heisei 31, but if your clock is set to May 2019 or later, June 1, 2019 is "correctly" shown as in Reiwa 1. This would be true even if the running Calculator and Windows code was already updated with the knowledge that the new era would be called "Reiwa." (Though the date when Reiwa would begin was specified by a law passed in 2017, Reiwa's name was only announced on April 1, 2019.)

(I forget whether this problem was solved in the calculator app itself or whether it inherited the solution from Windows' date and time formatting code.)

The justification was that the change of era would only be finalized when Akihito actually abdicated. In modern times, Akihito's abdication was the first time that the era changed without the reigning emperor dying. Calculator acting as if it knew before May 1, 2019 that Reiwa would begin on that day would thus be in bad taste, because it would be the moral equivalent of Calculator predicting or wishing for Akihito's death on April 30 - never mind that it was already well known that Akihito planned to abdicate on that day instead.

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No, the Japanese imperial calendar oddness is that it's otherwise exactly the Gregorian calendar, but the era may change midyear.