code: public static GsonBuilder registerInterval(GsonBuilder builder)
    {
      if (builder == null) { throw new NullPointerException("builder cannot be null"); }
  
      builder.registerTypeAdapter(INTERVAL_TYPE, new IntervalConverter());
  
      return builder;
    }
docs: Registers the {@link Interval} converter.
  @param builder The GSON builder to register the converter with.
  @return A reference to {@code builder}.