Error "gson.JsonSyntaxException" when I try to retrieve a document with a date in MongoDB

I receive from server a Json Object with a long value (Timestamp) for Date.

I am creating my Gson object:

final Gson gson = new GsonBuilder().registerTypeAdapter(Date.class, new JsonDateDeserializer()).create();

This is code for my JsonDateDeserializer:

 import com.google.gson.JsonDeserializationContext;
 import com.google.gson.JsonDeserializer;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonParseException;
 import java.lang.reflect.Type;
 import java.util.Date;

    public class JsonDateDeserializer implements JsonDeserializer<Date> {
    public Date deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        long time = json.getAsJsonPrimitive().getAsLong();
        Date d = new Date(time);
        return d;
    }
}

Updated long time = json.getAsJsonObject().getAsJsonPrimitive("$date").getAsLong();

 import com.google.gson.JsonDeserializationContext;
 import com.google.gson.JsonDeserializer;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonParseException;
 import java.lang.reflect.Type;
 import java.util.Date;

 public class JsonDateDeserializer implements JsonDeserializer<Date> {
      public Date deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
           long time = json.getAsJsonObject().getAsJsonPrimitive("$date").getAsLong();
           Date d = new Date(time);
           return d;
      }
 }

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