Just make sure to multiply by the right factor:
Micro: multiply by 1000000 (which is 10 power 6)
Milli: multiply by 1000 (which is 10 power 3)
This is what it should look like in Dart:
var date = new DateTime.fromMicrosecondsSinceEpoch(timestamp * 1000000);
Or
var date = new DateTime.fromMillisecondsSinceEpoch(timestamp * 1000);