Temporal.PlainMonthDay.prototype.day

The day accessor property of Temporal.PlainMonthDay instances returns a positive integer representing the 1-based day index in the month of this date, which is the same day number you would see on a calendar. It is calendar-dependent.

The set accessor of day is undefined. You cannot change this property directly. Use the with() method to create a new Temporal.PlainMonthDay object with the desired new value.

For general information and more examples, see Temporal.PlainDate.prototype.day.

Examples

undefined

Using day

js
const md = Temporal.PlainMonthDay.from("07-01"); // ISO 8601 calendar
console.log(md.day); // 1

const md2 = Temporal.PlainMonthDay.from("2021-07-01[u-ca=chinese]");
console.log(md2.day); // 22; it is May 22 in the Chinese calendar

Changing day

js
const md = Temporal.PlainMonthDay.from("07-01");
const newMD = md.with({ day: 15 });
console.log(newMD.toString()); // 07-15

By default, with() constrains the day to the range of valid values. So you can use { day: 1 } to set the day to the first day of the month, even if the first day does not have the number 1. Similarly, the following will set the day to the last day of the month:

js
const md = Temporal.PlainMonthDay.from("07-01");
const lastMD = md.with({ day: Number.MAX_VALUE }); // 07-31

For the purpose of PlainMonthDay, February is always considered to have 29 days.

js
const md = Temporal.PlainMonthDay.from("02-01");
const lastMD = md.with({ day: Number.MAX_VALUE }); // 02-29
console.log(lastMD.day); // 29

For other calendars, as long as there exists a year in which the month-day is valid, the month-day is considered valid, and the underlying reference year may therefore change. For example:

js
const md = Temporal.PlainMonthDay.from({
  monthCode: "M02",
  day: 29,
  calendar: "hebrew",
});
console.log(md.toString()); // 1972-11-06[u-ca=hebrew]
console.log(md.toLocaleString("en-US", { calendar: "hebrew" })); // 29 Heshvan
const lastMD = md.with({ day: Number.MAX_VALUE });
// 30 Heshvan does not exist in 1972, so the reference year changes to 1971
console.log(lastMD.toString()); // 1971-11-18[u-ca=hebrew]
console.log(lastMD.toLocaleString("en-US", { calendar: "hebrew" })); // 30 Heshvan

Specifications

See also