A Temporal.PlainYearMonth represents a particular month on the calendar.
For example, it could be used to represent a particular instance of a monthly recurring event, like "the June 2019 meeting".
Temporal.PlainYearMonth refers to the whole of a specific month; if you need to refer to a calendar event on a certain day, use Temporal.PlainDate or even Temporal.PlainDateTime.
A Temporal.PlainYearMonth can be converted into a Temporal.PlainDate by combining it with a day of the month, using the toPlainDate() method.
Parameters:
isoYear (number): A year.isoMonth (number): A month, ranging between 1 and 12 inclusive.calendar (optional string, Temporal.Calendar instance, or plain object): A calendar to project the month into.referenceISODay (optional for ISO 8601 calendar; required for other calendars): A reference day, used for disambiguation when implementing calendar systems.
For the ISO 8601 calendar, this parameter will default to 1 if omitted.
For other calendars, the must set this parameter to the ISO-calendar day corresponding to the first day of the desired calendar year and month.The
calendarandreferenceISODayparameters should be avoided becauseequalsorcomparewill considernew Temporal.PlainYearMonth(2000, 3, 'iso8601', 14)andPlainYearMonth(2000, 3, 'iso8601', 1)unequal even though they refer to the same year and month. When creating instances for non-ISO-8601 calendars (except when implementing a custom calendar) use thefrom()method which will automatically set a valid andequals-compatible reference day.
NOTE: To avoid infinite recursion,
referenceISODayis accepted as-is without validating that the day provided is actually the first day of the month in the desired calendar system. This lack of validation means thatequalsorcomparemay returnfalseforTemporal.PlainYearMonthinstances where the year and month and day are identical, but the reference days don't match. For this reason, it is STRONGLY recommended that this constructor SHOULD NOT be used except when implementing a custom calendar or when only using the ISO 8601 calendar. For other calendars, useTemporal.PlainYearMonth.from()which will automatically set the correct always set thereferenceISODayto the first of the month when constructing the new object.
Returns: a new Temporal.PlainYearMonth object.
All values are given as reckoned in the ISO 8601 calendar.
Together, isoYear, isoMonth, and referenceISODay must represent a valid date in that calendar, even if you are passing a different calendar as the calendar parameter.
The range of allowed values for this type is exactly enough that calling toPlainYearMonth() on any valid Temporal.PlainDate will succeed.
If isoYear and isoMonth are outside of this range, then this function will throw a RangeError.
Usually calendar will be a string containing the identifier of a built-in calendar, such as 'islamic' or 'gregory'.
Use an object if you need to supply custom calendar behaviour.
NOTE: The
isoMonthargument ranges from 1 to 12, which is different from legacyDatewhere months are represented by zero-based indices (0 to 11).
Usage examples:
// The June 2019 meeting
ym = new Temporal.PlainYearMonth(2019, 6);
// => 2019-06Parameters:
thing: The value representing the desired month.options (optional object): An object with properties representing options for constructing the date.
The following options are recognized:overflow (string): How to deal with out-of-range values if thing is an object.
Allowed values are constrain and reject.
The default is constrain.Returns: a new Temporal.PlainYearMonth object.
This static method creates a new Temporal.PlainYearMonth object from another value.
If the value is another Temporal.PlainYearMonth object, a new object representing the same month is returned.
If the value is any other object, it must have year (or era and eraYear), month (or monthCode) properties, and optionally a calendar property.
A Temporal.PlainYearMonth will be constructed from these properties.
If the calendar property is not present, it's assumed to be 'iso8601' (identifying the ISO 8601 calendar).
In this calendar, era is ignored.
Any non-object value is converted to a string, which is expected to be in ISO 8601 format. Any parts of the string other than the year and the month are optional and will be ignored.
If the string isn't valid according to ISO 8601, then a RangeError will be thrown regardless of the value of overflow.
A RangeError will also be thrown for strings that contain a Z in place of a numeric UTC offset, because interpreting these strings as a local date is usually a bug.
The overflow option works as follows, if thing is an object:
constrain mode (the default), any out-of-range values are clamped to the nearest in-range value.reject mode, the presence of out-of-range values will cause the function to throw a RangeError.The overflow option is ignored if thing is a string.
Additionally, if the result is earlier or later than the range of dates that Temporal.PlainYearMonth can represent (approximately half a million years centered on the Unix epoch), then this method will throw a RangeError regardless of overflow.
NOTE: The allowed values for the
thing.monthproperty start at 1, which is different from legacyDatewhere months are represented by zero-based indices (0 to 11).
Example usage:
ym = Temporal.PlainYearMonth.from('2019-06'); // => 2019-06
ym = Temporal.PlainYearMonth.from('2019-06-24'); // => 2019-06
ym = Temporal.PlainYearMonth.from('2019-06-24T15:43:27'); // => 2019-06
ym = Temporal.PlainYearMonth.from('2019-06-24T15:43:27+01:00[Europe/Brussels]');
// => 2019-06
ym === Temporal.PlainYearMonth.from(ym); // => false
ym = Temporal.PlainYearMonth.from({ year: 2019, month: 6 }); // => 2019-06
ym = Temporal.PlainYearMonth.from(Temporal.PlainDate.from('2019-06-24'));
// => 2019-06
// (same as above; Temporal.PlainDate has year and month properties)
// Different overflow modes
ym = Temporal.PlainYearMonth.from({ year: 2001, month: 13 }, { overflow: 'constrain' });
// => 2001-12
ym = Temporal.PlainYearMonth.from({ year: 2001, month: 13 }, { overflow: 'reject' });
// => throwsParameters:
one (Temporal.PlainYearMonth or value convertible to one): First month to compare.two (Temporal.PlainYearMonth or value convertible to one): Second month to compare.Returns: −1, 0, or 1.
Compares two Temporal.PlainYearMonth objects.
Returns an integer indicating whether one comes before or after or is equal to two.
one comes before twoone and two start on the same date when projected into the ISO 8601 calendarone comes after twoIf one and two are not Temporal.PlainYearMonth objects, then they will be converted to one as if they were passed to Temporal.PlainYearMonth.from().
Comparison is based on the first day of the month in the real world, regardless of the calendar.
For example, this method returns 0 for months that start on the same day in the ISO 8601 calendar, even if their calendars describe that day with a different year and/or month.
This function can be used to sort arrays of Temporal.PlainYearMonth objects.
For example:
one = Temporal.PlainYearMonth.from('2006-08');
two = Temporal.PlainYearMonth.from('2015-07');
three = Temporal.PlainYearMonth.from('1930-02');
sorted = [one, two, three].sort(Temporal.PlainYearMonth.compare);
sorted.join(' '); // => '1930-02 2006-08 2015-07'The above read-only properties allow accessing the year or month individually.
year is a signed integer representing the number of years relative to a calendar-specific anchor date.
For calendars that use eras, the anchor is usually aligned with the latest era so that eraYear === year for all dates in that era.
However, some calendars like Japanese may use a different anchor.month is a positive integer representing the ordinal index of the month in the current year.
For calendars like Hebrew or Chinese that use leap months, the same-named month may have a different month value depending on the year.
The first month in every year has month equal to 1.
The last month of every year has month equal to the monthsInYear property.
month values start at 1, which is different from legacy Date where months are represented by zero-based indices (0 to 11).monthCode is a calendar-specific string that identifies the month in a year-independent way.
For common (non-leap) months, monthCode should be `M${month}`, where month is zero padded up to two digits.
For uncommon (leap) months in lunisolar calendars like Hebrew or Chinese, the month code is the previous month's code with an "L" suffix appended.
Examples: 'M02' => February; 'M08L' => repeated 8th month in the Chinese calendar; 'M05L' => Adar I in the Hebrew calendar.Either month or monthCode can be used in from or with to refer to the month.
Similarly, in calendars that user eras an era/eraYear pair can be used in place of year when calling from or with.
Usage examples:
ym = Temporal.PlainYearMonth.from('2019-06');
ym.year; // => 2019
ym.month; // => 6
ym.monthCode; // => 'M06'
ym = Temporal.PlainYearMonth.from('2019-02-23[u-ca=hebrew]');
ym.year; // => 5779
ym.month; // => 6
ym.monthCode; // => 'M05L'The calendarId read-only property gives the identifier of the calendar that the year, month, and monthCode properties are interpreted in.
If yearMonth was created with a custom calendar object, this gives the id property of that project.
In calendars that use eras, the era and eraYear read-only properties can be used together to resolve an era-relative year.
Both properties are undefined when using the ISO 8601 calendar.
As inputs to from or with, era and eraYear can be used instead of year.
Unlike year, eraYear may decrease as time proceeds because some eras (like the BCE era in the Gregorian calendar) count years backwards.
ym = Temporal.PlainYearMonth.from('-000015-01-01[u-ca=gregory]');
ym.era;
// => 'bce'
ym.eraYear;
// => 16
ym.year;
// => -15The daysInMonth read-only property gives the number of days in the month.
This is 28, 29, 30, or 31, depending on the month and whether the year is a leap year.
Usage example:
// Attempt to write some mnemonic poetry
const monthsByDays = {};
for (let month = 1; month <= 12; month++) {
const ym = Temporal.PlainYearMonth.from({ year: 2020, calendar: 'iso8601', month });
monthsByDays[ym.daysInMonth] = (monthsByDays[ym.daysInMonth] || []).concat(ym);
}
const strings = monthsByDays[30].map((ym) => ym.toLocaleString('en', { month: 'long', calendar: 'iso8601' }));
// Shuffle to improve poem as determined empirically
strings.unshift(strings.pop());
const format = new Intl.ListFormat('en');
const poem = `Thirty days hath ${format.format(strings)}`;
console.log(poem);The daysInYear read-only property gives the number of days in the year that the month falls in.
This is 365 or 366, depending on whether the year is a leap year.
Usage example:
ym = Temporal.PlainYearMonth.from({ year: 2019, month: 6, calendar: 'iso8601' });
percent = ym.daysInMonth / ym.daysInYear;
`${ym.toLocaleString('en', {month: 'long', year: 'numeric', calendar: 'iso8601' })} was ${percent.toLocaleString('en', {style: 'percent'})} of the year!`
// => 'June 2019 was 8% of the year!'The monthsInYear read-only property gives the number of months in the year that the month falls in.
For the ISO 8601 calendar, this is always 12, but in other calendar systems it may differ from year to year.
Usage example:
ym = Temporal.PlainYearMonth.from('1900-01');
ym.monthsInYear; // => 12The inLeapYear read-only property tells whether the year that the date falls in is a leap year or not.
Its value is true if the year is a leap year, and false if not.
NOTE: A "leap year" is a year that contains more days than other years (for solar or lunar calendars) or more months than other years (for lunisolar calendars like Hebrew or Chinese). In the ISO 8601 calendar, a year is a leap year (and has exactly one extra day, February 29) if it is evenly divisible by 4 but not 100 or if it is evenly divisible by 400.
Usage example:
// Was June 2019 in a leap year?
ym = Temporal.PlainYearMonth.from('2019-06');
ym.inLeapYear; // => false
// Is 2100 a leap year? (no, because it's divisible by 100 and not 400)
ym.with({ year: 2100 }).inLeapYear; // => falseParameters:
yearMonthLike (object): an object with some or all of the properties of a Temporal.PlainYearMonth.options (optional object): An object with properties representing options for the operation.
The following options are recognized:overflow (string): How to deal with out-of-range values.
Allowed values are constrain and reject.
The default is constrain.Returns: a new Temporal.PlainYearMonth object.
This method creates a new Temporal.PlainYearMonth which is a copy of yearMonth, but any properties present on yearMonthLike override the ones already present on yearMonth.
Since Temporal.PlainYearMonth objects each represent a fixed year and month, use this method instead of modifying one.
If the result is earlier or later than the range of dates that Temporal.PlainYearMonth can represent (approximately half a million years centered on the Unix epoch), then this method will throw a RangeError regardless of overflow.
NOTE: The allowed values for the
yearMonthLike.monthproperty start at 1, which is different from legacyDatewhere months are represented by zero-based indices (0 to 11).
NOTE:
calendarandtimeZoneproperties are not allowed onyearMonthLike. It is not possible to convert aTemporal.PlainYearMonthto another calendar system without knowing the day of the month. If you need to do this, useyearMonth.toPlainDate({ day }).withCalendar(calendar).toPlainYearMonth().
Usage example:
ym = Temporal.PlainYearMonth.from('2019-06');
// Get December of that year
ym.with({ month: 12 }); // => 2019-12Parameters:
duration (Temporal.Duration or value convertible to one): The duration to add.options (optional object): An object with properties representing options for the addition.
The following options are recognized:overflow (string): How to deal with additions that result in out-of-range values.
Allowed values are constrain and reject.
The default is constrain.Returns: a new Temporal.PlainYearMonth object which is the month indicated by yearMonth plus duration.
This method adds duration to yearMonth, returning a month that is in the future relative to yearMonth.
The duration argument is an object with properties denoting a duration, such as { months: 5 }, or a string such as P5M, or a Temporal.Duration object.
If duration is not a Temporal.Duration object, then it will be converted to one as if it were passed to Temporal.Duration.from().
If duration has any units smaller than months, they will be treated as if they are being added to the first moment of the month given by yearMonth.
Effectively, this means that adding things like { days: 1 } will be ignored.
If the result is earlier or later than the range of dates that Temporal.PlainYearMonth can represent (approximately half a million years centered on the Unix epoch), then this method will throw a RangeError regardless of overflow.
The overflow option has no effect in the default ISO 8601 calendar, because a year is always 12 months and therefore not ambiguous.
It doesn't matter in this case that years and months can be different numbers of days, as the resolution of Temporal.PlainYearMonth does not distinguish days.
However, overflow may have an effect in other calendars where years can be different numbers of months.
Adding a negative duration is equivalent to subtracting the absolute value of that duration.
Usage example:
ym = Temporal.PlainYearMonth.from('2019-06');
ym.add({ years: 20, months: 4 }); // => 2039-10Parameters:
duration (Temporal.Duration or value convertible to one): The duration to subtract.options (optional object): An object with properties representing options for the subtraction.
The following options are recognized:overflow (string): How to deal with additions that result in out-of-range values.
Allowed values are constrain and reject.
The default is constrain.Returns: a new Temporal.PlainYearMonth object which is the month indicated by yearMonth minus duration.
This method subtracts duration from yearMonth, returning a month that is in the future relative to yearMonth.
The duration argument is an object with properties denoting a duration, such as { months: 5 }, or a string such as P5M, or a Temporal.Duration object.
If duration is not a Temporal.Duration object, then it will be converted to one as if it were passed to Temporal.Duration.from().
If duration has any units smaller than months, they will be treated as if they are being subtracted from the last moment of the month given by yearMonth.
Effectively, this means that subtracting things like { days: 1 } will be ignored.
If the result is earlier or later than the range of dates that Temporal.PlainYearMonth can represent (approximately half a million years centered on the Unix epoch), then this method will throw a RangeError regardless of overflow.
The overflow option has no effect in the default ISO 8601 calendar, because a year is always 12 months and therefore not ambiguous.
It doesn't matter in this case that years and months can be different numbers of days, as the resolution of Temporal.PlainYearMonth does not distinguish days.
However, overflow may have an effect in other calendars where years can be different numbers of months.
Subtracting a negative duration is equivalent to adding the absolute value of that duration.
Usage example:
ym = Temporal.PlainYearMonth.from('2019-06');
ym.subtract({ years: 20, months: 4 }); // => 1999-02Parameters:
other (Temporal.PlainYearMonth or value convertible to one): Another month until when to compute the difference.options (optional object): An object with properties representing options for the operation.
The following options are recognized:largestUnit (string): The largest unit of time to allow in the resulting Temporal.Duration object.
Valid values are 'auto', 'year' and 'month'.
The default is 'auto'.smallestUnit (string): The smallest unit of time to round to in the resulting Temporal.Duration object.
Valid values are 'year' and 'month'.
The default is 'month', i.e., no rounding.roundingIncrement (number): The granularity to round to, of the unit given by smallestUnit.
The default is 1.roundingMode (string): How to handle the remainder, if rounding.
Valid values are 'ceil', 'floor', 'expand', 'trunc', 'halfCeil', 'halfFloor', 'halfExpand', 'halfTrunc', and 'halfEven'.
The default is 'trunc', which truncates any remainder towards zero.Returns: a Temporal.Duration representing the elapsed time after yearMonth and until other.
This method computes the difference between the two months represented by yearMonth and other, optionally rounds it, and returns it as a Temporal.Duration object.
If other is earlier than yearMonth then the resulting duration will be negative.
If using the default options, adding the returned Temporal.Duration to yearMonth will yield other.
If other is not a Temporal.PlainYearMonth object, then it will be converted to one as if it were passed to Temporal.PlainYearMonth.from().
The largestUnit option controls how the resulting duration is expressed.
The returned Temporal.Duration object will not have any nonzero fields that are larger than the unit in largestUnit.
A difference of one year and two months will become 14 months when largestUnit is "months", for example.
However, a difference of one month will still be one month even if largestUnit is "years".
A value of 'auto' means 'year'.
You can round the result using the smallestUnit, roundingIncrement, and roundingMode options.
These behave as in the Temporal.Duration.round() method, but increments of months and larger are allowed.
Because rounding to calendar units requires a reference point, the first day of yearMonth is used as the starting point.
The default is to do no rounding.
Unlike other Temporal types, weeks and lower are not allowed for either largestUnit or smallestUnit, because the data model of Temporal.PlainYearMonth doesn't have that accuracy.
Computing the difference between two months in different calendar systems is not supported.
Usage example:
ym = Temporal.PlainYearMonth.from('2006-08');
other = Temporal.PlainYearMonth.from('2019-06');
ym.until(other); // => P12Y10M
ym.until(other, { largestUnit: 'month' }); // => P154M
other.until(ym, { largestUnit: 'month' }); // => -P154M
// If you really need to calculate the difference between two YearMonths
// in days, you can eliminate the ambiguity by explicitly choosing the
// day of the month (and if applicable, the time of that day) from which
// you want to reckon the difference. For example, using the first of
// the month to calculate a number of days:
ym.toPlainDate({ day: 1 }).until(other.toPlainDate({ day: 1 }), { largestUnit: 'day' }); // => P4687DParameters:
other (Temporal.PlainYearMonth or value convertible to one): Another month since when to compute the difference.options (optional object): An object with properties representing options for the operation.
The following options are recognized:largestUnit (string): The largest unit of time to allow in the resulting Temporal.Duration object.
Valid values are 'auto', 'year' and 'month'.
The default is 'auto'.smallestUnit (string): The smallest unit of time to round to in the resulting Temporal.Duration object.
Valid values are 'year' and 'month'.
The default is 'month', i.e., no rounding.roundingIncrement (number): The granularity to round to, of the unit given by smallestUnit.
The default is 1.roundingMode (string): How to handle the remainder, if rounding.
Valid values are 'ceil', 'floor', 'expand', 'trunc', 'halfCeil', 'halfFloor', 'halfExpand', 'halfTrunc', and 'halfEven'.
The default is 'trunc', which truncates any remainder towards zero.Returns: a Temporal.Duration representing the elapsed time before yearMonth and since other.
This method computes the difference between the two months represented by yearMonth and other, optionally rounds it, and returns it as a Temporal.Duration object.
If other is later than yearMonth then the resulting duration will be negative.
This method is similar to Temporal.PlainYearMonth.prototype.until(), but reversed.
If using the default options, subtracting the returned Temporal.Duration from yearMonth will yield other, and ym1.since(ym2) will yield the same result as ym1.until(ym2).negated().
Usage example:
ym = Temporal.PlainYearMonth.from('2019-06');
other = Temporal.PlainYearMonth.from('2006-08');
ym.since(other); // => P12Y10MParameters:
other (Temporal.PlainYearMonth or value convertible to one): Another month to compare.Returns: true if yearMonth and other are equal, or false if not.
Compares two Temporal.PlainYearMonth objects for equality.
This function exists because it's not possible to compare using yearMonth == other or yearMonth === other, due to ambiguity in the primitive representation and between Temporal types.
If other is not a Temporal.PlainYearMonth object, then it will be converted to one as if it were passed to Temporal.PlainYearMonth.from().
Note that this function will return false if the two objects have different calendar properties, even if the actual years and months are equal.
If you don't need to know the order in which the two months occur, then this function may be less typing and more efficient than Temporal.PlainYearMonth.compare.
Example usage:
ym = Temporal.PlainYearMonth.from('2019-06');
other = Temporal.PlainYearMonth.from('2006-08');
ym.equals(other); // => false
ym.equals(ym); // => trueParameters:
options (optional object): An object with properties influencing the formatting.
The following options are recognized:calendarName (string): Whether to show the calendar annotation in the return value.
Valid values are 'auto', 'always', 'never', and 'critical'.
The default is 'auto'.Returns: a string in the ISO 8601 date format representing yearMonth.
This method overrides the Object.prototype.toString() method and provides a convenient, unambiguous string representation of yearMonth.
The string can be passed to Temporal.PlainYearMonth.from() to create a new Temporal.PlainYearMonth object.
Normally, a calendar annotation is shown when yearMonth's calendar is not the ISO 8601 calendar.
By setting the calendarName option to 'always' or 'never' this can be overridden to always or never show the annotation, respectively.
Normally not necessary, a value of 'critical' is equivalent to 'always' but the annotation will contain an additional ! for certain interoperation use cases.
For more information on the calendar annotation, see the Temporal string formats documentation.
Example usage:
ym = Temporal.PlainYearMonth.from('2019-06');
ym.toString(); // => '2019-06'Parameters:
locales (optional string or array of strings): A string with a BCP 47 language tag with an optional Unicode extension key, or an array of such strings.options (optional object): An object with properties influencing the formatting.Returns: a language-sensitive representation of yearMonth.
This method overrides Object.prototype.toLocaleString() to provide a human-readable, language-sensitive representation of yearMonth.
The locales and options arguments are the same as in the constructor to Intl.DateTimeFormat.
The calendar in the output locale (given by new Intl.DateTimeFormat(locales, options).resolvedOptions().calendar) must match monthDay.calendar, or this method will throw an exception.
This is because it's not possible to convert a Temporal.PlainMonthDay from one calendar to another without more information.
In order to ensure that the output always matches monthDay's internal calendar, you must either explicitly construct monthDay with the locale's calendar, or explicitly specify the calendar in the options parameter:
yearMonth.toLocaleString(locales, { calendar: yearMonth.calendar });
// OR
yearMonth = Temporal.PlainYearMonth.from({ /* ... */, calendar: localeCalendar });
yearMonth.toLocaleString();Example usage:
({ calendar } = new Intl.DateTimeFormat().resolvedOptions());
ym = Temporal.PlainYearMonth.from({ year: 2019, month: 6, calendar });
ym.toLocaleString(); // example output: '6/2019'
// Same as above, but explicitly specifying the calendar:
ym.toLocaleString(undefined, { calendar });
ym.toLocaleString('de-DE', { calendar }); // example output: '6.2019'
ym.toLocaleString('de-DE', { month: 'long', year: 'numeric', calendar }); // => 'Juni 2019'
ym.toLocaleString(`en-US-u-nu-fullwide-ca-${calendar}`); // => '６/２０１９'Returns: a string in the ISO 8601 date format representing yearMonth.
This method is the same as yearMonth.toString().
It is usually not called directly, but it can be called automatically by JSON.stringify().
The reverse operation, recovering a Temporal.PlainYearMonth object from a string, is Temporal.PlainYearMonth.from(), but it cannot be called automatically by JSON.parse().
If you need to rebuild a Temporal.PlainYearMonth object from a JSON string, then you need to know the names of the keys that should be interpreted as Temporal.PlainYearMonths.
In that case you can build a custom "reviver" function for your use case.
Example usage:
const boardMeeting = {
id: 4,
agenda: ['Roll call', 'Budget'],
meetingYearMonth: Temporal.PlainYearMonth.from({ year: 2019, month: 3 })
};
const str = JSON.stringify(boardMeeting, null, 2);
console.log(str);
// =>
// {
// "id": 4,
// "agenda": [
// "Roll call",
// "Budget"
// ],
// "meetingYearMonth": "2019-03"
// }
// To rebuild from the string:
function reviver(key, value) {
if (key.endsWith('YearMonth')) return Temporal.PlainYearMonth.from(value);
return value;
}
JSON.parse(str, reviver);This method overrides Object.prototype.valueOf() and always throws an exception.
This is because it's not possible to compare Temporal.PlainYearMonth objects with the relational operators <, <=, >, or >=.
Use Temporal.PlainYearMonth.compare() for this, or yearMonth.equals() for equality.
Parameters:
day (object): An object with a 'day' property, which must be a valid day of yearMonth.Returns: a Temporal.PlainDate object that represents the calendar date of day in yearMonth.
This method can be used to convert Temporal.PlainYearMonth into a Temporal.PlainDate, by supplying a calendar day to use.
The converted object carries a copy of all the relevant fields of yearMonth.
Usage example:
ym = Temporal.PlainYearMonth.from('2019-06');
ym.toPlainDate({ day: 24 }); // => 2019-06-24Returns: a Temporal.Calendar instance or plain object representing the calendar in which yearMonth is reckoned.
This method is mainly useful if you need an object on which to call calendar methods. Most code will not need to use it.
Returns: a plain object with properties expressing yearMonth in the ISO 8601 calendar, as well as the calendar (usually a string, but may be an object) that yearMonth is reckoned in.
This method is mainly useful if you are implementing a custom calendar. Most code will not need to use it.
The value of the isoDay property will be equal to the referenceISODay constructor argument passed when yearMonth was constructed.
Usage example:
ym = Temporal.PlainYearMonth.from('2019-06');
ym.getISOFields().isoYear; // => 2019