`round` on date-having types should support month and year rounding

round() with smallestUnit: 'month' and smallestUnit: 'year' should be allowed on PlainDate, PlainDateTime, ZonedDateTime, and (year only) PlainYearMonth.

In V1, PlainDate lacks a round method, and PlainDateTime and ZonedDateTime are both limited to smallestUnit: 'day' or smaller.

Rounding up would advance to the first day of the next month or year, respectively. (Not the last day of the month or year.)

roundingIncrement would be limited to 1 or undefined, because the number of months in a year can vary in lunisolar calendars like Hebrew or Chinese that have 12 months in non-leap years but 13 months in leap years.

Advantages:

Better consistency across Temporal types

Easier support for use cases like 'closest start of year'

/** Are we halfway done with the year yet? */
function isHalfwayDoneWithYear(date) {
return date.round({ smallestUnit: 'year' }).year === date.year;
}
// current workaround
function isHalfwayDoneWithYear(date) {
const daysSoFar = date.since(date.with({month: 1, day: 1}));
return daysSoFar >= date.daysInYear / 2;
}

Avoids bugs in non-ISO calendars, e.g.

// proposed
date = date.round({smallestUnit: 'month', roundingMode: 'ceil'});
// current, which breaks in the chinese or hebrew calendar where years can have 12 or 13 months
if (date.day > 1) {
date = date.month === 12 ? date.with({ year: date.year + 1, month: 1 }) : date.with({ month: date.month + 1 });
}
// BTW, here's a less obvious alternative to buggy code above
if (date.day > 1) date = date.add({month: 1}).with({day: 1})

Concerns:

Workaround is pretty easy today, so not sure this change is necessarily needed. Workaround is:

function roundToMonthsOrYears(temporalInstance, roundOptions) { 
  const baseValues = { 
    year: temporalInstance.year, month: 1, monthCode: 'M01', day: 1,
    hour: 0, minute: 0, second: 0, millisecond: 0, microsecond: 0, nanosecond: 0
  };
  const base = temporalInstance.with(baseValues)
  const duration = base.until(temporalInstance, roundOptions);
  return base.add(duration);
}
roundToMonthsOrYears(Temporal.PlainDate.from('2021-02-15'), { smallestUnit: 'month', roundingMode: 'trunc' });
// => 2021-02-01
roundToMonthsOrYears(Temporal.PlainDate.from('2021-02-15'), { smallestUnit: 'month', roundingMode: 'ceil' });
// => 2021-03-01
roundToMonthsOrYears(Temporal.PlainDate.from('2021-02-15'), { smallestUnit: 'month', roundingMode: 'halfExpand' });
// => 2021-03-01
roundToMonthsOrYears(Temporal.PlainDate.from('2021-02-14'), { smallestUnit: 'month', roundingMode: 'halfExpand' });
// => 2021-02-01

Prior art:

Year and month rounding already exists on until and since and Duration.prototype.round, just not on the round methods of PlainDate, PlainDateTime, ZonedDateTime.

See https://github.com/tc39/proposal-temporal/issues/1785 for previous discussion.

Constraints / corner cases:

None that I know of, but I can add if they're raised in comments.

js-temporal / proposal-temporal-v2