Date.prototype.toLocaleString()

const event = new Date(Date.UTC(2012, 11, 20, 3, 0, 0));

// British English uses day-month-year order and 24-hour time without AM/PM
console.log(event.toLocaleString("en-GB", { timeZone: "UTC" }));
// Expected output: "20/12/2012, 03:00:00"

// Korean uses year-month-day order and 12-hour time with AM/PM
console.log(event.toLocaleString("ko-KR", { timeZone: "UTC" }));
// Expected output: "2012. 12. 20.  3:00:00"

toLocaleString()
toLocaleString(locales)
toLocaleString(locales, options)

const date = new Date(Date.UTC(2012, 11, 12, 3, 0, 0));

console.log(date.toLocaleString());
// "12/11/2012, 7:00:00 PM" if run in en-US locale with time zone America/Los_Angeles

function toLocaleStringSupportsLocales() {
  return (
    typeof Intl === "object" &&
    !!Intl &&
    typeof Intl.DateTimeFormat === "function"
  );
}

const date = new Date(Date.UTC(2012, 1, 2, 3, 0, 0));

// Formats below assume the local time zone of the locale;
// America/Los_Angeles for the US

// US English uses month-day-year order and 12-hour time with AM/PM
console.log(date.toLocaleString("en-US"));
// "2/1/2012, 7:00:00 PM" (UTC-8 is the previous day)

// British English uses day-month-year order and 24-hour time without AM/PM
console.log(date.toLocaleString("en-GB"));
// "02/02/2012, 03:00:00" (UTC+0 or UTC+1 depending on time of the year)

// Korean uses year-month-day order and 12-hour time with AM/PM
console.log(date.toLocaleString("ko-KR"));
// "2012. 2. 2.  12:00:00"

// Arabic in most Arabic-speaking countries uses Eastern Arabic numerals
console.log(date.toLocaleString("ar-EG"));
// "// :: "

// For Japanese, applications may want to use the Japanese calendar,
// where 2012 was the year 24 of the Heisei era
console.log(date.toLocaleString("ja-JP-u-ca-japanese"));
// "H24/2/2 12:00:00"

// When requesting a language that may not be supported, such as
// Balinese, include a fallback language (in this case, Indonesian)
console.log(date.toLocaleString(["ban", "id"]));
// "2/2/2012 11.00.00"

const date = new Date(Date.UTC(2012, 11, 20, 3, 0, 0));

// Request a weekday along with a long date
const options = {
  weekday: "long",
  year: "numeric",
  month: "long",
  day: "numeric",
};
console.log(date.toLocaleString("de-DE", options));
// Example output: "Donnerstag, 20. Dezember 2012"
// The exact date may shift depending on your local time zone.

// An application may want to use UTC and make that visible
options.timeZone = "UTC";
options.timeZoneName = "short";
console.log(date.toLocaleString("en-US", options));
// Example output: "Thursday, December 20, 2012 at UTC"

// Sometimes even the US needs 24-hour time
console.log(date.toLocaleString("en-US", { hour12: false }));
// Example output: "12/19/2012, 19:00:00"
// The exact date and time may shift depending on your local time zone.