my-website/static/scripts/calcage.js

"use strict";

{
  // Month index must be two because counting begins at 0 (for January)
  const myBirthDate = new Date(2001, 2, 29);
  const today = new Date();

  let yearsSinceBirth = today.getFullYear() - myBirthDate.getFullYear();

  // Correct yearsSinceBirth if between New Year and my birthday
  if (today.getMonth() < myBirthDate.getMonth()) yearsSinceBirth--;
  else if (
    today.getMonth() == myBirthDate.getMonth() &&
    today.getDate() < myBirthDate.getDate()
  )
    yearsSinceBirth--;

  document.getElementById("age-display").innerText = yearsSinceBirth.toString();
}