Gson dateformat to parse/output unix-timestamps

Creating a custom DateTypeAdapter was the way to go.

MyDateTypeAdapter

public class MyDateTypeAdapter extends TypeAdapter<Date> {
    @Override
    public void write(JsonWriter out, Date value) throws IOException {
        if (value == null) {
            out.nullValue();
            return;
        }
        out.value(value.getTime() / 1000);
    }

    @Override
    public Date read(JsonReader in) throws IOException {
        if (in == null) {
            return null;
        }
        return new Date(in.nextLong() * 1000);
    }
}

Don't forget to register it!

MyDateTypeAdapter myAdapter = new MyDateTypeAdapter();
Gson gson = new GsonBuilder().registerTypeAdapter(Date.class, myAdapter).create();