code: public DateTimePrinter toPrinter() {
          Object f = getFormatter();
          if (isPrinter(f)) {
              InternalPrinter ip = (InternalPrinter) f;
              return InternalPrinterDateTimePrinter.of(ip);
          }
          throw new UnsupportedOperationException("Printing is not supported");
      }
docs: Internal method to create a DateTimePrinter instance using all the
  appended elements.
  <p>
  Most applications will not use this method.
  If you want a printer in an application, call {@link #toFormatter()}
  and just use the printing API.
  <p>
  Subsequent changes to this builder do not affect the returned printer.
  
  @throws UnsupportedOperationException if printing is not supported