[delayed]

Most applications don't have to worry about it, but platforms do. The SQL standard requires accurate date representation for years 0000-9999. I was surprised to learn about the Gregorian shift while testing our database system. I also learned about "war time" in WW2 and the "battle of the clocks" in colonial India. A lot of history came out of debugging those tests.

Library and Archival management systems

Lots of written work pre 1582 that needs to be accurately catalogued.

Source: I used to work on one of the first computerised library management software systems, and plenty of our academic customers had manuscripts dating back that far.

> Who writes computer programs that deal with pre-1582 _dates_ at all?

People writing knowledge base software, like Wikidata. It's helpful to be able to display historical dates in any desired calendar, and also to be able to do date arithmetic between any two arbitrary dates.

This point about using seconds is entirely correct, and date-time should always be in epoch timestamp (numeric) except for human interfaces (input or rendering for output). APIs and web-forms should never pass multipart date strings because they can be internally inconsistent, for example February 30. (edit: unless there's no sequential meaning in the date and the date is just a local day name without the notion that adding or subtracting 1 takes you to yesterday or tomorrow (https://gist.github.com/timvisee/fcda9bbdff88d45cc9061606b4b...)).

(p.s. autarch thanks for your efforts in open source).

I've been writing some programs to deal with the Maya Calendars, where it is an issue.

On some level, it doesn't matter if an inscription corresponds to "August 31, 83 CE" or "September 2, 83 CE" (a date potentially inscribed at the site Takalik Abaj in Proleptic Gregorian or Julian), but it's nice to try to get the details correct. And if you want to correlate with other events in history (especially if you can find astronomical event data from Europe), you need to be at least consistent.

Plus, it's just fun to think about these events from long ago in terms (dates) we understand.

Astronomy

Genealogy apps, e.g. Ancestry.com