DateTimeTypeConverter.java

import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonParseException;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;
import java.lang.reflect.Type;
import org.joda.time.DateTime;

public class DateTimeTypeConverter implements JsonSerializer<DateTime>, JsonDeserializer<DateTime> {

  @Override
  public JsonElement serialize(DateTime src, Type srcType, JsonSerializationContext context) {
    return new JsonPrimitive(src.toString());
  }

  @Override
  public DateTime deserialize(JsonElement json, Type type, JsonDeserializationContext context)
      throws JsonParseException {

    // Do not try to deserialize null or empty values
    if (json.getAsString() == null || json.getAsString().isEmpty()) {
      return null;
    }
    long milliseconds = json.getAsLong() * 1000l;
    return new DateTime(milliseconds);
  }
}

JodaDateTimeConverterFactory.java

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import org.joda.time.DateTime;
import retrofit2.converter.gson.GsonConverterFactory;

public class JodaDateTimeConverterFactory {

  private static Gson gson() {
    GsonBuilder gsonBuilder = new GsonBuilder();
    gsonBuilder.registerTypeAdapter(DateTime.class, new DateTimeTypeConverter());
    return gsonBuilder.create();
  }

  /**
   * Create an instance using {@code gson} for conversion. The provided {@code gson} instance has a
   * registered TypeAdapter to handle deserializing {@link DateTime}. Encoding to JSON and
   * decoding from JSON (when no charset is specified by a header) will use UTF-8.
   */
  public static GsonConverterFactory create() {
    return GsonConverterFactory.create(gson());
  }
}

RestClient.java

/**
 * For REST over HTTP(S). Holds the client for other services to put interfaces against.
 */
public class RestClient {

  private final Retrofit mClient;

  public RestClient() {
    mClient = new Retrofit.Builder()
        .baseUrl(baseUrl())
        .addConverterFactory(JodaDateTimeConverterFactory.create())
        .build();
  }

  /**
   * Creates an implementation of the API defined by the {@link }
   */
  public <T> T create(Class<T> apiInterfaceClass) {
    return mClient.create(apiInterfaceClass);
  }
}