code: public static String getCustomID(String id) {
          int[] fields = new int[4];
          if (parseCustomID(id, fields)) {
              return formatCustomID(fields[1], fields[2], fields[3], fields[0] < 0);
          }
          return null;
      }
docs: Parse a custom time zone identifier and return the normalized
  custom time zone identifier for the given custom id string.
  @param id a string of the form GMT[+-]hh:mm, GMT[+-]hhmm, or
  GMT[+-]hh.
  @return The normalized custom id string.