JavaScript 날짜 확인 코드가 \'mm/dd/yyyy\' 형식을 올바르게 확인하지 못하는 이유는 무엇입니까?

<code class="js">function isDate(ExpiryDate) {
  var objDate, // date object initialized from the ExpiryDate string
    mSeconds, // ExpiryDate in milliseconds
    day, // day
    month, // month
    year; // year
  // date length should be 10 characters (no more no less)
  if (ExpiryDate.length !== 10) {
    return false;
  }
  // third and sixth character should be '/'
  if (ExpiryDate.substring(2, 3) !== '/' || ExpiryDate.substring(5, 6) !== '/') {
    return false;
  }
  // extract month, day and year from the ExpiryDate (expected format is mm/dd/yyyy)
  // subtraction will cast variables to integer implicitly (needed
  // for !== comparing)
  month = ExpiryDate.substring(0, 2) - 1; // because months in JS start from 0
  day = ExpiryDate.substring(3, 5) - 0;
  year = ExpiryDate.substring(6, 10) - 0;
  // test year range
  if (year < 1000 || year > 3000) {
    return false;
  }
  // convert ExpiryDate to milliseconds
  mSeconds = new Date(year, month, day).getTime();
  // initialize Date() object from calculated milliseconds
  objDate = new Date();
  objDate.setTime(mSeconds);
  // compare input date and parts from Date() object
  // if difference exists then date isn't valid
  if (
    objDate.getFullYear() !== year ||
    objDate.getMonth() !== month ||
    objDate.getDate() !== day
  ) {
    return false;
  }
  // otherwise return true
  return true;
}</code>

<code class="js">// extract year
if (ExpiryDate.substring(6, 7) === '/') {
  year = parseInt(ExpiryDate.substring(6, 8), 10); // 2-digit year
} else {
  year = parseInt(ExpiryDate.substring(6, 10), 10); // 4-digit year
}
// test year range
if (year < 1000 || year > 2999) {
  return false;
}</code>