Japanese calendar and eraDisplay

[sffc]

I think this is reasonable except for the splitting of era and year. In Japan, the current year is either 2022 or R4 (令和4年) but not just 4(年). Beyond just awkwardness, this would have the potential of incurring confusion between calendar years and year-length durations (as, depending on context, 4年 could also mean "4 years").

Originally posted by @rkirsling in #7 (comment)

[sffc]

CC @Louis-Aime

[sffc]

Discussion: the decision for whether to display the era with "auto" may depend on:

1. Calendar system
2. Locale
3. Current date

However, the spec currently only allows differentiation on (3). The spec should be updated to allow all three.

The same problem may also come up in other contexts such as whether or not to display relatedISO. The same sort of model can be applied in the future.

[Louis-Aime]

As of "today" (2023-12), the decision for whether to display the era (when "year" shall be displayed) already depends on calendar and the language and possibly region tags of the locale. In certain cases, the author cannot change that decision.

One objective of the proposal is to give back some power to the author. The other main objective is to make the decision to display the era depend on the date displayed. I think we have to make clear all these objectives, by adding values for the option:

• never: Do not display era, even if year is displayed. The author shall be aware that this may lead to confusion, e.g. the reader could miss that a year under the ethiopic calendar should be counted from "Amete Alem" (World's Creation), not from Incarnation, if he is not aware of the context. After all, Intl accepts combinations of options where month is missing although year and day are present.
• discreet: display era only if (1) year is displayed, and (2) era is necessary to understand the year when using this calendar; e.g. display era only for B.C. dates with gregory; with japanese you would probably always display the era.
• explicit: display era if (and only if) year is displayed.
• always: always display era, even if year shall not be displayed.
• auto (or void): depending on locale and calendar, discreet or explicit is chosen. e.g. th-TH-u-ca-gregory you take explicit because 'th' 's default calendar use the same dates as gregory but not the same year origin; as a simple rule, discreet is the default value, and explicit is used if the calendar is not the locale's default.

[sffc]

TG2 discussion: https://github.com/tc39/ecma402/blob/master/meetings/notes-2023-12-14.md#eradisplay-updates