round duration to closer second, for a countdown clock - https://dartpad.dartlang.org/2a08161c5f889e018938316237c0e810

main.dart

void main() {
  Duration duration = new Duration(minutes: 1, seconds: 59, milliseconds: 99);

  print(secondRounder(duration));
}

// Rounds Duration to seconds based on milliseconds
// This is semi-equivalent to a non-existing `Duration.ceil(milliseconds)`
Duration secondRounder(Duration duration) {
  int roundedDuration;

  if (duration.inMilliseconds > (duration.inSeconds * 1000)) {
    roundedDuration = duration.inSeconds + 1;
  } else {
    roundedDuration = duration.inSeconds;
  }

  return new Duration(seconds: roundedDuration);
}