code: public TimeZoneFormat setGMTZeroFormat(String gmtZeroFormat) {
          if (isFrozen()) {
              throw new UnsupportedOperationException("Attempt to modify frozen object");
          }
          if (gmtZeroFormat == null) {
              throw new NullPointerException("Null GMT zero format");
          }
          if (gmtZeroFormat.length() == 0) {
              throw new IllegalArgumentException("Empty GMT zero format");
          }
          _gmtZeroFormat = gmtZeroFormat;
          return this;
      }
docs: Sets the localized GMT format string for GMT(UTC) itself (GMT offset is 0).
  
  @param gmtZeroFormat the localized GMT format string for GMT(UTC).
  @return this object.
  @throws UnsupportedOperationException when this object is frozen.
  @see #getGMTZeroFormat()