Retrofit 2 Android Deserializer JSON GSON

I just started with Retrofit 2 for Android, I opted for the good reviews I had, although I would have stayed with Volley since I see it much easier.

The fact is that the issue of deserializar the JSON with GSON is giving me quite the can.

I put the code so that it is understood:

I have created a Singleton to manage the connections of the whole app, I hope it's okay:

public class SingletonRetrofit {

    public static final String BASE_URL = "xxxxxxxxxxxxxxxxxxx";
    private static Retrofit retrofit = null;

    public static Retrofit getClient() {

        Gson gson = new GsonBuilder()
                        .registerTypeAdapter(User.class, new GenericDeserializer<User>())
                        .create();

        if (retrofit == null) {
            retrofit = new Retrofit.Builder()
                    .baseUrl(BASE_URL)
                    .addConverterFactory(GsonConverterFactory.create(gson))
                    .build();
        }
        return retrofit;
    }

}

Then I created the following "generic" deserializer, and now I explain because I understand, but here I would like you to correct me if I'm wrong:

    public class GenericDeserializer<T> implements JsonDeserializer<T>
{
    @Override
    public T deserialize(JsonElement je, Type type, JsonDeserializationContext jdc)
            throws JsonParseException
    {
        // Get the "content" element from the parsed JSON
        JsonElement content = je.getAsJsonObject().get("type");


        if(content.toString().equals("\"dbUserResponse\"")){
            content = je.getAsJsonObject().get("user");

            JsonObject jsonObject = content.getAsJsonObject();
            if(jsonObject.has("nacimiento")){
                String date = jsonObject.get("nacimiento").getAsString();

                SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
                Date date2 = null;
                try {
                    date2 = sdf.parse(date);
                } catch (ParseException e) {
                    e.printStackTrace();
                }
                String newstring = new SimpleDateFormat("yyyy-MM-dd").format(date2);
                jsonObject.addProperty("nacimiento", newstring);
                content = jsonObject.getAsJsonObject();  
                }
            }

        }
        if(content.toString().equals("\"dbAlertResponse\"")){
        // other case..... etc...

        }

        else{
            //content = '{"error":"mierror"}';
        }


        // Deserialize it. You use a new instance of Gson to avoid infinite recursion
        // to this deserializer
        return new Gson().fromJson(content, type);

    }
}

And now where I want to call my API:

//METODO CON RETROFIT
        pDialog = new ProgressDialog(this);
        pDialog.setMessage("Cargando...");
        pDialog.show();
        final String TAG = RegisterUser.class.getSimpleName();


        ApiInterface apiService = SingletonRetrofit.getClient().create(ApiInterface.class);


        Call<User> call = apiService.getUser(1);
        call.enqueue(new Callback<User>() {
            @Override
            public void onResponse(Call<User> call, retrofit2.Response<User> response) {
                User userdatos = response.body();

                if(userdatos != null){
                    ((EditText) findViewById(R.id.email)).setText(userdatos.getEmail());
                    ((EditText) findViewById(R.id.dateBirth)).setText(userdatos.getNacimiento().toString());

                }
                else{
                    //Toast.makeText(PersonalProfileActivity.this, "Error al cargar usuario", Toast.LENGTH_LONG).show();
                }
                pDialog.hide();

            }

            @Override
            public void onFailure(Call<User>call, Throwable t) {
                // Log error here since request failed
                Log.e(TAG, "error a la M");
                Log.e(TAG, t.toString());
            }
        });

Well, I expose my problems:

1st - I can not format a date that arrives in the following JSON:

{
  "type": "dbUserResponse",
  "user": {
    "email": "[email protected]",
    "nacimiento": "1995-01-01T00:00:00+01:00",
  }
}

All the answers of the calls come to me in this "style". That is, "type" indicates what type of response it is. For example in the case of "alerts" I would get "type": "dbAlertResponse".

For this reason I have created a generic deserializer, but I do not know if it is the right thing to do. In addition, as I say, it is impossible for the date format that is coming to me to be converted to the one I am interested in, which would be "yyyy-MM-dd".

2nd - I tried to add another deserializerTypeAdapter in the creation of the gson, but either I do not know how it is done, or it has not worked for me.

How would this situation be resolved correctly? thanks