Normative: Don't add default formatting to lone era

[ptomato]

Previously, if era was given by itself in the formatting options, the default formatting for the other date/time components would be added. This is inconsistent with what we do for dayPeriod where a lone dayPeriod produces formats such as "in the afternoon".

The use case of obtaining the localized era name has also been mentioned several times.

This change makes a lone era be passed to the format matcher algorithm without added date/time components, and requires implementations to have at least a year-month-day-era and year-month-era format available (which looks to be widely available across languages in CLDR.)

See: #461

[anba]

Does this require any changes in https://tc39.es/ecma402/#sec-intl.datetimeformat-internal-slots? Right now neither era nor timeZoneName are required to be present in the minimum available formats. That means for example when requesting {era: "long"}, it's valid for BasicFormatMatcher to return the format for "hour, minute" (assuming only the minimum required formats are present in the implementation.)

When given the request {era: "long"}, BasicFormatMatcher will compute the best score for:

• year, month
• month, day
• hour, minute
• dayPeriod, hour

There's a removal penalty because era is not present, and twice an addition penalty, because other fields are present. So the best score is -120 - 2*20 = -160. The formats list is unordered, so any of these four formats can be returned.

[ptomato]

I assumed no, because we also permit dayPeriod by itself but dayPeriod by itself is not part of the minimum required formats. (Although, maybe this is slightly different because dayPeriod is present with other fields.)

[anba]

(Although, maybe this is slightly different because dayPeriod is present with other fields.)

Yes, this is the important difference. Standalone dayPeriod is not required to be an available format, but at least it's guaranteed that a format is returned which includes dayPeriod when requesting standalone dayPeriod. Standalone era or standalone timeZoneName can return formats which contain neither era nor timeZoneName.

[ptomato]

Conclusion in the TG2 meeting today was:

• This should be split up into two PRs and era added separately from timeZoneName
• In both cases research should be done to check if it is likely to be web compatible
• If not web compatible, consider other options like { era: 'long', standalone: true }

[ptomato]

I've updated this to modify only era. I will open a separate PR for timeZoneName.