The Temporal.PlainYearMonth object represents the year and month of a calendar date, without a day or time zone; for example, an event on a calendar that happens during the whole month. It is fundamentally represented as an ISO 8601 calendar date, with year, month, and day fields, and an associated calendar system. The day is used to disambiguate the year-month in non-ISO calendar systems.

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The valueOf() method of Temporal.PlainMonthDay instances throws a TypeError, which prevents Temporal.PlainMonthDay instances from being implicitly converted to primitives when used in arithmetic or comparison operations.

The withCalendar() method of Temporal.PlainDateTime instances returns a new Temporal.PlainDateTime object representing this date-time interpreted in the new calendar system. Because all Temporal objects are designed to be immutable, this method essentially functions as the setter for the date-time's calendarId property.

The toJSON() method of Temporal.PlainYearMonth instances returns a string representing this year-month in the same RFC 9557 format as calling toString(). It is intended to be implicitly called by JSON.stringify().

The calendarId accessor property of Temporal.PlainDate instances returns a string representing the calendar used to interpret the internal ISO 8601 date.

The equals() method of Temporal.PlainMonthDay instances returns true if this month-day is equivalent in value to another month-day (in a form convertible by Temporal.PlainMonthDay.from()), and false otherwise. They are compared both by their underlying ISO date values and their calendars.

The calendarId accessor property of Temporal.PlainMonthDay instances returns a string representing the calendar used to interpret the internal ISO 8601 date.

The monthCode accessor property of Temporal.PlainDateTime instances returns a calendar-specific string representing the month of this date. It is calendar-dependent.

The Temporal.Duration object represents a difference between two time points, which can be used in date/time arithmetic. It is fundamentally represented as a combination of years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, and nanoseconds values.