I don't think that's the problem; I think that's the one thing that's not a problem. Monotonic time is not that hard. The hardest thing about it is just convincing programmers to use it, and making sure your libraries use it properly. I don't want to say it's trivial, but it's not that hard once both you and your code base internalize "convert everything to internal UTC representation as quickly as possible, convert to local time as late as possible on the way out".

It's everything else that's a problem. It's the places where you input the Japanese time, and there's an era that the code has never heard of for some old system. There is no way for that system to even conceivably handle it without some sort of update, at least a database update. They're talking about creating a new character for the era, which literally no system on Earth can currently use because it doesn't even exist yet, and, again, anything old that can't be updated can't conceivably display it correctly. They might be able to display it as the individual characters, but they still won't know that's a date, or that they have to. As mipmap04 says, it's where Japanese dates were stored as a string or something, because goodness knows I've seen enough US dates stored as strings.

And inferring from the article that the tax authority is going to extend the old era, I assume that at the very least a good chunk of the government works on Japanese dates, so it's going to be important not to, say, print tax forms that have � in the date, etc.