Correctly handle ISO reference year for MonthDay

[Manishearth]

https://tc39.es/proposal-temporal/#sec-temporal-calendarmonthdaytoisoreferencedate

2. Return an implementation-defined ISO Date Record, or throw a RangeError exception, as described below.

Currently we return (iso era, 1972) for all calendars.

I'm not quite sure what we ought to do here.

[Manishearth]

I think the standard ought to specify a default Temporal year, much like it specifies a default Temporal epoch

cc @sffc

[Manishearth]

We also have this issue:

temporal/src/builtins/core/calendar.rs

Lines 244 to 246 in 3fa989f

	// TODO: This may get complicated...
	// For reference: https://github.com/tc39/proposal-temporal/blob/main/polyfill/lib/calendar.mjs#L1275.
	Err(TemporalError::range().with_message("Not yet implemented/supported."))

@nekevss the line number for that link has changed; is there something subtle we need to worry about?

I wonder if there is sensible default behavior we can pick here that works for all calendars while we wait for official word from the spec? Curious if @sffc has thoughts.

[nekevss]

Huh, I'm not remembering the exact point on that file.

There wasn't anything particularly subtle being noted. It was mainly the amount of month focused methods involved in non-ISO calendars.

[Manishearth]

Wrote up some options in tc39/proposal-intl-era-monthcode#60 (comment)

[Manishearth]

We now handle most of these according to the spec algorithm except for Chinese/Dangi. I'd like to wait for unicode-org/icu4x#6800 before attempting Chinese/Dangi.

[sffc]

Thought: with the proleptic approximation, we can choose years far in the past and fully cover the 30-day leap months.

[Manishearth]

@sffc My understanding is that most attested Chinese tabular (algorithmic) calendar approximations like pingqi apparently don't cover all leap months. I can't remember my source on this, unfortunately.

I know that Le calendrier chinois: structure et calculs has details on this, I'll check it out later.

It's worth considering: we have an aversion to using out-of-range dates (I certianly do!) but the actual reference year for MonthDay doesn't really matter: it only shows up when formatting in strange scenarios, I think. So we might be fine using something far out of range.