code: public TimeZoneFormat setGMTOffsetDigits(String digits) {
          if (isFrozen()) {
              throw new UnsupportedOperationException("Attempt to modify frozen object");
          }
          if (digits == null) {
              throw new NullPointerException("Null GMT offset digits");
          }
          String[] digitArray = toCodePoints(digits);
          if (digitArray.length != 10) {
              throw new IllegalArgumentException("Length of digits must be 10");
          }
          _gmtOffsetDigits = digitArray;
          return this;
      }
docs: Sets the decimal digit characters used for localized GMT format.
  
  @param digits a string contains the decimal digit characters from 0 to 9 n the ascending order.
  @return this object.
  @throws IllegalArgumentException when the string did not contain ten characters.
  @throws UnsupportedOperationException when this object is frozen.
  @see #getGMTOffsetDigits()