lib/jsdom/living/helpers/dates-and-times.js
function parseYearlessDateString(str) {
  const matches = yearlessDateRe.exec(str);
  if (!matches) {
    return null;
  }
  const month = Number(matches[1]);
  if (month < 1 || month > 12) {
    return null;
  }
  const day = Number(matches[2]);
  if (day < 1 || day > numberOfDaysInMonthOfYear(month, 4)) {
    return null;
  }
  return { month, day };
}
function parseYearlessDateString(str) {
  const matches = yearlessDateRe.exec(str);
  if (!matches) {
    return null;
  }
  const month = Number(matches[1]);
  if (month < 1 || month > 12) {
    return null;
  }
  const day = Number(matches[2]);
  if (day < 1 || day > numberOfDaysInMonthOfYear(month, 4)) {
    return null;
  }
  return { month, day };
}
https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#parse-a-yearless-date-string