Yeah, just run it and see how large a billion actually is!

billion.dart

void main() {
  final million = 1e6;
  final billion = 1e9;

  final mDur = Duration(seconds: million.toInt());
  final bDur = Duration(seconds: billion.toInt());

  print('1 million seconds is\n${durToString(mDur)}');
  print("");
  print('1 billion seconds is\n${durToString(bDur)}');
}

String durToString(Duration d) {
  // A Years, B Months, C Days, D Hours, E Minutes, F Seconds
  final years = d.inDays ~/ 365;
  final months = (d.inDays % 365) ~/ 30;
  final days = (d.inDays % 365) % 30;
  final hours = d.inHours % 24;
  final minutes = d.inMinutes % 60;
  final seconds = d.inSeconds % 60;

  final yStr = years > 0 ? '$years years' : '';
  final mStr = months > 0 ? '$months months' : '';
  final dStr = days > 0 ? '$days days' : '';
  final hStr = hours > 0 ? '$hours hours' : '';
  final minStr = minutes > 0 ? '$minutes minutes' : '';
  final secStr = seconds > 0 ? '$seconds seconds' : '';

  final parts = [yStr, mStr, dStr, hStr, minStr, secStr];
  final nonEmptyParts = parts.where((p) => p.isNotEmpty).toList();

  if (nonEmptyParts.length == 1) {
    return nonEmptyParts.first;
  }

  final lastPart = nonEmptyParts.removeLast();

  return '${nonEmptyParts.join(', ')} and $lastPart';
}

Run it, and then you'll see:

1 million seconds is
11 days, 13 hours, 46 minutes and 40 seconds

1 billion seconds is
31 years, 8 months, 19 days, 1 hours, 46 minutes and 40 seconds

That's it.