Q:

Why does Spring Boot change the format of a JSON response even when a custom converter which never handles JSON is configured?

I encountered the problem when I try to answer the question:
Request with multipart/form-data returns 415 error
According to the document about HttpMessageConverters(Spring Boot Features 7.1.2. HttpMessageConverters), we can add a custom HttpMessageConverter by configuring a bean of HttpMessageConverters. I see a JSON response that I don't expected when I configure a custom converter. 

I created a simple end point that returns a JSON consists of Date and LocalDate fields.

When I don't configure any custom converter, I get them in String format. 

{
   "date": "2020-02-29T06:13:45.875+0000",
   "localDate": "2020-02-29"
}

When I configure a custom converter which does NOT handle JSON responses, I get them in Number format.

{
    "date": 1582956902270,
    "localDate": [
        2020,
        2,
        29
    ]
}

My questions are:

Why is the format of the JSON response changed even when I configure a custom converter never handling JSON responses?
How to avoid it? I want to make Spring Boot handle JSON responses in the same way that it does when I don't configure custom converters.

[Sample Application]
Spring Boot Version: 2.2.5.RELEASE
@RestController
@SpringBootApplication
public class MyApplication {

    @GetMapping("/test")
    public SomeDates test() {
        return new SomeDates(new Date(), LocalDate.now());
    }

    @Data
    public class SomeDates {
        private final Date date;
        private final LocalDate localDate;
    }

    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

@Configuration
public class MyConfigurer {

    @Bean
    public HttpMessageConverters customConverters() {
        return new HttpMessageConverters(new ConverterDoingNothingForTestPurpose());
    }

    public static class ConverterDoingNothingForTestPurpose implements HttpMessageConverter<Object> {

        @Override
        public boolean canRead(Class<?> clazz, MediaType mediaType) {
            return false;
        }

        @Override
        public boolean canWrite(Class<?> clazz, MediaType mediaType) {
            return false;
        }

        @Override
        public List<MediaType> getSupportedMediaTypes() {
            return Collections.emptyList();
        }

        @Override
        public Object read(Class<?> clazz, HttpInputMessage inputMessage)
                throws IOException, HttpMessageNotReadableException {
            throw new RuntimeException("This converter should not be invoked.");
        }

        @Override
        public void write(Object t, MediaType contentType, HttpOutputMessage outputMessage)
                throws IOException, HttpMessageNotWritableException {
            throw new RuntimeException("This converter should not be invoked.");
        }
    }
}

A:

By declaring your own HttpMessageConverters bean, you have stopped Spring MVC from being configured with Spring Boot’s converters. Crucially in this case, this leaves Spring MVC using its default JSON converter rather than Spring Boot’s. This is why the JSON response has changed.
If you want to add a custom converter to those that Spring Boot configured by default, remove your HttpMessageConverters bean and define a bean for your custom converter instead:
@Bean
ConverterDoingNothingForTestPurpose testConverter() {
    return new ConverterDoingNothingForTestPurpose();
}

Your custom converter will be picked up as an additional converter by the auto-configured HttpMessageConverters bean.

