code: public static long subMonths(final String dateString1, final String dateString2) {
  
      Date date1 = getDateFromString(dateString1, DEFAULT_DATE_SIMPLE_PATTERN);
      Date date2 = getDateFromString(dateString2, DEFAULT_DATE_SIMPLE_PATTERN);
      return subMonths(date1, date2);
    }
docs: Get how many months between two date, the date pattern is 'yyyy-MM-dd'.
  
  @param dateString1 date string to be tested.
  @param dateString2 date string to be tested.
  @return how many months between two date.
  @throws DateException