Fewer CalendarFields/DateFromFields calls during many PlainYearMonth ops

[arshaw]

For these PlainYearMonth operations...

• add
• subtract
• since
• until

...it essentially converts to a PlainDate by going to the start-of-month, then carries out its operation, then converts back to PlainYearMonth.

To convert to the start-of-month, it uses a very heavyweight set of operations:

• CalendarFields
• CalendarDateFromFields
• (and re-parse all fields previously parsed)

It's much more efficient to leverage the CalendarProtocol's day() method to move to the start-of-month. Here's what fullcalendar's polyfill does:

export function moveToMonthStart(
  calendarOps: { day: DayOp },
  isoFields: IsoDateFields,
): IsoDateFields {
  return moveByIsoDays(isoFields, 1 - calendarOps.day(isoFields))
}

I'll experiment with making the reference polyfill do this. I'll open a PR...

[ptomato]

Long story, but I'm not sure we can assume that moveByIsoDays(isoFields, 1 - calendarOps.day(isoFields)) is equivalent to .with({day: 1}). See #1315.

tl;dr ICU may in the future get a historically accurate Julian-Gregorian calendar, which skips 10 days in October 1582. (Or some later date, depending on how quickly the jurisdiction adopted the calendar reform.) The day() method of the Calendar protocol gives the calendar number of the day, not the 1-based index of the day. So October 15, 1582 is only 4 days from the start of the month, not 14 days.

This isn't a concern with any current builtin calendar, but we decided that we need to be robust against it.

[arshaw]

Wow, I'll add that to the list of strange things I've learned about calendar systems!

Makes sense that .with() would need to be used for bulletproofing.

Closing this issue.

[justingrant]

I'll add that to the list of strange things I've learned about calendar systems!

@arshaw You may want to take a look at these sections in the docs:

• https://tc39.es/proposal-temporal/docs/calendar.html#invariants-across-calendars
• https://tc39.es/proposal-temporal/docs/calendar.html#writing-cross-calendar-code

Feel free to PR additions/edits to those docs if you learn something strange about calendars that isn't represented there!