Incorrect conversion between calendars

[devongovett]

When converting dates before 1582 to other Gregorian-derived calendars such as buddhist and japanese, the result is incorrect.

Temporal.PlainDate.from({year: 1582, month: 1, day: 1}).withCalendar('buddhist').month
// => 12 (incorrect)

As discussed in tc39/ecma402#1003, all of these calendars should be defined as proleptic - meaning they extend before the Gregorian Calendar was invented in 1582. Therefore, only the year should be changed - the month and day should return 1 in this example.

This is related to browser issues across all JS engines in Intl.DateTimeFormat. Firefox has changed their behavior in 139 beta to match their native Temporal implementation which now returns the correct result.

Temporal.PlainDate.from({year: 1582, month: 1, day: 1}).withCalendar('buddhist').month
// => 1 (correct)

[justingrant]

Hi Devon, thanks for the bug report. (love your work, btw!)

FYI @ptomato, the comment below implies that we worked around this JS engine bug, but polyfill calendar code has changed a lot since I wrote it several years ago and I couldn't find the the "custom calendarToIsoDate implementation" that this comment is talking about. So I assume it was removed somewhere between then and now. Does this change sound familiar?

temporal-polyfill/lib/calendar.ts

Lines 760 to 769 in 91a5feb

	const calculateSameMonthResult = (diffDays: number) => {
	// If the estimate is in the same year & month as the target, then we can
	// calculate the result exactly and short-circuit any additional logic.
	// This optimization assumes that months are continuous. It would break if
	// a calendar skipped days, like the Julian->Gregorian switchover. But
	// current ICU calendars only skip days (japanese/roc/buddhist) because of
	// a bug (https://bugs.chromium.org/p/chromium/issues/detail?id=1173158)
	// that's currently worked around by a custom calendarToIsoDate
	// implementation in those calendars. So this optimization should be safe
	// for all ICU calendars.

FYI @arshaw

[ptomato]

@justingrant As best I can tell, that is probably mistakenly named and is actually referring to isoToCalendarDate https://github.com/js-temporal/temporal-polyfill/blob/main/lib/calendar.ts#L1852-L1859

[arshaw]

Also in fullcalendar's temporal-polyfill implementation. Makes sense that it would be buggy because it scrapes from Intl.DTF. Might workaround. Thx @justingrant