if (Temporal.Now.plainDateISO().dayOfYear === 1) {
  console.log("Happy New Year");
}

Formatting a date with toLocaleString()

const birthday = Temporal.PlainMonthDay.from("09-27");
const birthdayIn2025 = birthday.toPlainDate({ year: 2025 });
console.log(birthdayIn2025.toLocaleString('en-GB', {
  weekday: "long",
  year: "numeric",
  month: "long",
  day: "numeric",
}));
// Saturday, 27 September 2025

HTML date inputs

<input type="datetime-local"> is not timezone aware.

Browser support