The toPlainDateTime() method of Temporal.PlainDate instances returns a new Temporal.PlainDateTime object representing this date and a supplied time in the same calendar system.
toPlainDateTime()
toPlainDateTime(plainTime)plainTime OptionalA string, an object, or a Temporal.PlainTime instance representing the time component of the resulting PlainDateTime. It is converted to a Temporal.PlainTime object using the same algorithm as Temporal.PlainTime.from(). Defaults to "00:00:00".
A new Temporal.PlainDateTime object representing the date and time specified by this date and plainTime, interpreted in the calendar system of this date.
const date = Temporal.PlainDate.from("2021-07-01");
const dateTime = date.toPlainDateTime("12:34:56");
console.log(dateTime.toString()); // 2021-07-01T12:34:56
const midnight = date.toPlainDateTime();
console.log(midnight.toString()); // 2021-07-01T00:00:00
const date2 = Temporal.PlainDate.from("2021-07-01[u-ca=chinese]");
const dateTime2 = date2.toPlainDateTime("12:34:56");
console.log(dateTime2.toString()); // 2021-07-01T12:34:56[u-ca=chinese]