As 1570051184 milliseconds is already 18 days, and multiplying that by 1000 would be 49 years, and @Alexander Stolte is less than half that age, I would check first what sort of time span we are expecting, to be sure, to be sure. ?
This is my source date:
2019-10-02 21:19:44 +0000
converting it with the timeIntervalSince1970 property gets:
1570051184.79484
and multiply by 1000 and converting it back to string gives me:
2019-10-02 23:19:44 +0200
it works