Temporal.PlainYearMonth

Table of Contents

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.

Constructor

new Temporal.PlainYearMonth(isoYear: number, isoMonth: number, calendar?: string | object, referenceISODay: number = 1) : Temporal.PlainYearMonth

Parameters:

The calendar and referenceISODay parameters should be avoided because equals or compare will consider new Temporal.PlainYearMonth(2000, 3, 'iso8601', 14) and PlainYearMonth(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 the from() method which will automatically set a valid and equals-compatible reference day.

NOTE: To avoid infinite recursion, referenceISODay is 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 that equals or compare may return false for Temporal.PlainYearMonth instances 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, use Temporal.PlainYearMonth.from() which will automatically set the correct always set the referenceISODay to 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.

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.

NOTE: The isoMonth argument ranges from 1 to 12, which is different from legacy Date where months are represented by zero-based indices (0 to 11).

Usage examples:

// The June 2019 meeting
ym = new Temporal.PlainYearMonth(2019, 6);
// => 2019-06

Static methods

Temporal.PlainYearMonth.from(thing: any, options?: object) : Temporal.PlainYearMonth

Parameters:

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 will be assumed to be Temporal.Calendar.from('iso8601'), 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.

The overflow option works as follows, if thing is an object:

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.month property start at 1, which is different from legacy Date where 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('2019-06-24T15:43:27Z'); // => 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' });
  // => throws

Temporal.PlainYearMonth.compare(one: Temporal.PlainYearMonth | object | string, two: Temporal.PlainYearMonth | object | string) : number

Parameters:

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.

If one and two are not Temporal.PlainYearMonth objects, then they will be converted to one as if they were passed to Temporal.PlainYearMonth.from().

Note that this function will not return 0 if the two objects have different calendar properties, even if the actual years and months are equal. If the months are equal, then .calendar.id will be compared lexicographically, in order to ensure a deterministic sort order.

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'

Properties

yearMonth.year : number

yearMonth.month : number

yearMonth.monthCode : string

The above read-only properties allow accessing the year or month individually.

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'

yearMonth.calendar : object

The calendar read-only property gives the calendar that the year and month properties are interpreted in.

yearMonth.era : string | undefined

yearMonth.eraYear : number | undefined

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;
// => -15

yearMonth.daysInMonth : number

The 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);

yearMonth.daysInYear : number

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!'

yearMonth.monthsInYear: number

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; // => 12

yearMonth.inLeapYear : boolean

The 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.

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; // => false

Methods

yearMonth.with(yearMonthLike: object, options?: object) : Temporal.PlainYearMonth

Parameters:

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.month property start at 1, which is different from legacy Date where months are represented by zero-based indices (0 to 11).

NOTE: calendar and timeZone properties are not allowed on yearMonthLike. It is not possible to convert a Temporal.PlainYearMonth to another calendar system without knowing the day of the month. If you need to do this, use yearMonth.toPlainDate({ day }).withCalendar(calendar).toPlainYearMonth().

Usage example:

ym = Temporal.PlainYearMonth.from('2019-06');
// Get December of that year
ym.with({ month: 12 }); // => 2019-12

yearMonth.add(duration: Temporal.Duration | object | string, options?: object) : Temporal.PlainYearMonth

Parameters:

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 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 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 }); // => 2039-10

yearMonth.subtract(duration: Temporal.Duration | object | string, options?: object) : Temporal.PlainYearMonth

Parameters:

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 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 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 }); // => 1999-02

yearMonth.until(other: Temporal.PlainYearMonth | object | string, options?: object) : Temporal.Duration

Parameters:

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. The returned Temporal.Duration, when added to yearMonth with the same options, 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' }); // => P4687D

yearMonth.since(other: Temporal.PlainYearMonth | object | string, options?: object) : Temporal.Duration

Parameters:

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. The returned Temporal.Duration, when subtracted from yearMonth using the same options, will yield other. Using default options, ym1.since(ym2) yields the same result as ym1.until(ym2).negated(), but results may differ with options like { largestUnit: 'month' }.

Usage example:

ym = Temporal.PlainYearMonth.from('2019-06');
other = Temporal.PlainYearMonth.from('2006-08');
ym.since(other); // => P12Y10M

yearMonth.equals(other: Temporal.PlainYearMonth | object | string) : boolean

Parameters:

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); // => true

yearMonth.toString() : string

Parameters:

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. For more information on the calendar annotation, see ISO string extensions.

Example usage:

ym = Temporal.PlainYearMonth.from('2019-06');
ym.toString(); // => '2019-06'

yearMonth.toLocaleString(locales?: string | array<string>, options?: object) : string

Parameters:

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'

yearMonth.toJSON() : string

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);

yearMonth.valueOf()

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.

yearMonth.toPlainDate(day: object) : Temporal.PlainDate

Parameters:

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 }); // => 2019-06-24

yearMonth.getISOFields(): { isoYear: number, isoMonth: number, isoDay: number, calendar: object }

Returns: a plain object with properties expressing yearMonth in the ISO 8601 calendar, as well as the value of yearMonth.calendar.

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