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Comment by autarch | Hacker News Reader

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

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.

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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.

For sure. I was thinking more of "normal" applications, as opposed to things like SQL databases. It looks like the SQL standard explicitly calls for the use of the Gregorian calendar for dates, so I think that answers the previous question of "Who else uses the proleptic Gregorian calendar other than programmers of datetime libraries?"

"normal applications", no. But there are more platforms besides databases, e.g. programming languages. Any platform that works with dates will have to deal with these weird quirks. And application that works with dates probably would not, although there's always leap seconds...

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).

For astronomical code (and any sane date code) you use Julian Day Number which is the number of days since a defined day 0.

The one most seen would be Microsoft DateTime first in Excel where day 1 is 31st Dec 1899 in USA/Georgian Calendar - but only useful from March 1st 1900

Wikipedia gives the Astronomical Day number as

The Julian day number (JDN) has the same epoch as the Julian period, but counts the number of days since the epoch rather than the number of years since then. Specifically, Julian day number 0 is assigned to the day starting at noon Universal Time on Monday, January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar).[4][5][6][a] For example, the Julian day number for the day starting at 12:00 UT (noon) on January 1, 2000, was 2451545.[7]

There is the Modified Julian Dates several of which including the Microsoft one are listed in Variants here https://en.wikipedia.org/wiki/Julian_day

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.

People working with buildings, infrastructure, and artifacts that predate 1582. Many such things in Europe

Genealogical software, historical video games (CK2 has at least one mod which adds alternative calendars), I can imagine there are some needs for this in contract management software for countries with very old businesses and properties like England and Japan.

> (CK2 has at least one mod which adds alternative calendars)

I imagine that must have been absolutely riveting to mod in.

Genealogy apps, e.g. Ancestry.com

Astronomy

That's its own fun rabbit hole.

https://en.wikipedia.org/wiki/Epoch_(astronomy)

https://en.wikipedia.org/wiki/Barycentric_Coordinate_Time

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