But it is typically used when referring to events located in areas with no calendar system familiar to modern readers, most notably the Maya calendar. Maya Long Count dates are typically converted to the proleptic Gregorian calendar.

Also, many modern historical writings will display both Julian and Gregorian dates, particularly for dates around when the calendars were changing or when context is unclear (e.g. a historical figure who was born in an area with one calendar and died in an area with another calendar).

Fun fact: Greece was still using the old Julian calendar as of 1928, so it's entirely possible that you'll see dates on relatively recent Greek documents that are up to 14 days off from the calendar the rest of the world was using. Civil timekeeping is just a mess.

In science-related code, I can image a case for calculating things like seconds elapsed across a very long time period. But in that case you're not really dealing with dates. Instead, you want something like the Unix epoch, but probably with an earlier epoch start.

There may be some very niche cases for pre-1582 dates in code that I'm not thinking of. But I'd expect that in this case you're going to put a lot of effort into finding the right libraries, or you might just write your own code.

Regardless, the linked blog post says of DateTime: "Unfortunately, like Python, a proper error message for impossible Gregorian dates is notably absent."

But this is just wrong. There's no "proper error message" to emit. The code works as documented.