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