Era codes for the Coptic calendar

[sffc]

For positive years, it is A.M. (Anno Martyrum). This is fairly well documented.

CLDR has an inverse Coptic era in data. I'm having trouble finding a third-party source that discusses this inverse era. There are a few translations for it in CLDR:

• French: avant Dioclétien ("before Diocletian")
• Romanian: înainte de Anno Martyrum ("before A.M.")
• A few others: https://www.unicode.org/cldr/charts/47/by_type/date_&_time.coptic.html

French abbreviates it as "a.D.", which means "Before Diocletian", not "After Diocletian"

Most articles I find about the Coptic calendar use the ISO Common Era or Julian eras when discussing things that happened before the Diocletian.

Do we have any more evidence?

CC @Louis-Aime

[Manishearth]

Do we know where the CLDR one came from? Perhaps the coptic calendar should have a singe era, we've already found the French data to be suspicious here.

My general experience with these calendars is that they are not used to talk about dates before their single era, so there is no common pattern there. Gregorian is somewhat unique in that, partly because it is a somewhat global standard which makes it liable to use proleptically: other calendars are used either in civil contexts (to talk about current dates, or near-modern important events) or religious contexts (to talk about current dates, and past dates relevant to the religion), but otherwise won't be used for arbitrary past dates. Julian used to be a similar global standard and has the same thing going for it. Japanese has eras in modern times which means it also uses them.

The Coptic church presumably talks about the birth of Jesus, but .. it's quite possible they refer to those dates with a different calendar.

The lack of actual proleptic usage for these calendars makes me prefer treating it as having a single era, ignoring the CLDR data.

cc @robertbastian

[sffc]

I'm okay if we want to propose that CLDR removes the inverse era and falls back to negative numbers.

[sffc]

Let's try to use this thread for Coptic era discussion.

@robertbastian said in #25 (comment):

I do want to point out that the Coptic calendar should have eras BD and AD, not BM and AM.

@gibson042 said in unicode-org/cldr#4519 (comment):

There's a suggestion at tc39/proposal-intl-era-monthcode#25 (comment):

the Coptic calendar should have eras BD and AD, not BM and AM

However, it's at least partly at odds with other resources I can find, all of which use "AM" or "A.M.":

• https://suscopts.org/coptic-orthodox/fasts-and-feasts/
• https://www.timeanddate.com/calendar/coptic-calendar.html
• https://en.orthodoxwiki.org/Coptic_Calendar#The_Coptic_year
• https://www.stvnashville.org/coptic-new-year#yui_3_17_2_1_1742572291617_643

I do see references to A.D. (from "anno Diocletiani"), but concrete usage seems rare. I have no opinion on which is more correct here.

[sffc]

I can't find any evidence in English of "AD" being used to mean "After Diocletian". I also can't find any evidence in English of any backwards era anchored on Diocletian.

This web site has a lot of dates in a lot of eras:

https://albertacoptic.org/index.php/coptic-calendar

The following eras all appear on this site:

• A.U.C., anchored at the traditional founding of Rome. Read more: https://en.wikipedia.org/wiki/Ab_urbe_condita
• B.C., as in the Julian era Before Christ
• A.D., as in the Julian era Anno Domini
• A.M., as in the Coptic era Anno Martyrum

The Wikipedia page on Anno Domini is interesting: https://en.wikipedia.org/wiki/Anno_Domini

I think given that A.D. is used in so many Coptic materials to refer to Anno Domini, we should definitely not use it for any other purpose.

[sffc]

ok in unicode-org/cldr#4519 I'm proposing "am" since that is definitely in use, and "bd" since I can't find any evidence of what else it should be.

It's not terribly different than AD being derived from Latin "Anno Domini" and BC being derived from English "Before Christ".

[Louis-Aime]

Together with @Manishearth , the lack of documented proleptic usage for the Coptic calendars makes me prefer treating it as having a single era, which we may code 'am' and name "Anno Martyrum" which is effectively used and not ambiguous with "AD".

[sffc]

TG2 discussion: https://github.com/tc39/ecma402/blob/main/meetings/notes-2025-04-03.md#era-codes-for-the-coptic-calendar

Main conclusion is that we prefer using something like "am-inverse" or "coptic-inverse" or "before-am" instead of "bd" for the era that does not exist but is in CLDR.

[robertbastian]

I'm confused by this conclusion. It seems like everyone was in favour of removing the era (I'm very much in favour as well), so why create and release yet another identifier instead of just removing the era?

[sffc]

Removing the era requires approval from CLDR, which we do not have at this time, so the agreement is basically for a stopgap era code if we end up shipping it

[robertbastian]

Doesn't the era code change in CLDR require the same approvals? Or is the stopgap to diverge from CLDR?

[sffc]

I'm proposing to remove the BD era in unicode-org/cldr#4620