The number of milliseconds since the "Unix epoch" 1970-01-01T00:00:00Z (UTC).
This value is independent of the time zone.
IO wrapper around dart DateTime.now().millisecondsSinceEpoch.
DateTime.now().millisecondsSinceEpoch
IO<int> get now => dateNow.map((date) => date.millisecondsSinceEpoch);