flutter-sport/lib/utils/date.dart

DateTime offsetDate(int type, int offset) {
  DateTime time = DateTime.now();

  DateTime next;

  if (type == 0) {
    next = time.add(Duration(days: offset));
  } else if (type == 1) {
    next = time.add(Duration(days: offset * 7));
  } else if (type == 2) {
    next = DateTime(time.year, time.month + offset, 1);
  } else {
    next = time.add(Duration(days: offset * 365 + 1));
  }

  return next;
}