code: static String createDateStringFromIntegerInput(
              @IntRange(from = 1, to = 12) int month,
              @IntRange(from = 0, to = 9999) int year) {
          String monthString = String.valueOf(month);
          if (monthString.length() == 1) {
              monthString = "0" + monthString;
          }
  
          String yearString = String.valueOf(year);
          // Three-digit years are invalid.
          if (yearString.length() == 3) {
              return "";
          }
  
          if (yearString.length() > 2) {
              yearString = yearString.substring(yearString.length() - 2);
          } else if (yearString.length() == 1) {
              yearString = "0" + yearString;
          }
  
          return monthString + yearString;
      }
docs: Creates a string value to be entered into an expiration date text field
  without a divider. For instance, (1, 2020) => "0120". It doesn't matter if
  the year is two-digit or four. (1, 20) => "0120".
  
  Note: A four-digit year will be truncated, so (1, 2720) => "0120". If the year
  date is 3 digits, the data will be considered invalid and the empty string will be returned.
  A one-digit date is valid (represents 2001, for instance).
  
  @param month a month of the year, represented as a number between 1 and 12
  @param year a year number, either in two-digit form or four-digit form
  @return a length-four string representing the date, or an empty string if input is invalid