The internet is telling me that Taiwan uses both calendars. Is that not true? Surely even your grandparents know that it's 2023 in the rest of the world.

It's irrelevant to my point though. Just because the standard is used by 99% of people instead of 100% doesn't mean programmers should invent a new one, for the same reason they chose January 1st 1970 and not November 15th 1970 or any other day of the year. There's only one obvious choice.