The toPlainDate() method of Temporal.PlainYearMonth instances returns a new Temporal.PlainDate object representing this year-month and a supplied day in the same calendar system.
toPlainDate(dayInfo)dayInfo OptionalAn object representing the day component of the resulting PlainDate, containing the following property:
dayCorresponds to the day property.
A new Temporal.PlainDate object representing the date specified by this year-month and the day in dayInfo, interpreted in the calendar system of this year-month.
RangeErrorThrown if any of the options is invalid.
TypeErrorThrown if dayInfo is not an object.
const ym = Temporal.PlainYearMonth.from("2021-07");
const date = ym.toPlainDate({ day: 1 });
console.log(date.toString()); // 2021-07-01
const ym2 = Temporal.PlainYearMonth.from("2021-07-01[u-ca=chinese]");
const date2 = ym2.toPlainDate({ day: 15 });
console.log(date2.toString()); // 2021-06-24[u-ca=chinese]