class Semester {
  Semester({required DateTime startDate, required this.totalWeeks})
    : startDate = DateTime.utc(startDate.year, startDate.month, startDate.day) {
    if (totalWeeks < 1 || totalWeeks > 30) {
      throw ArgumentError('学期周数应为 1–30');
    }
  }
  DateTime get firstMonday =>
      startDate.subtract(Duration(days: startDate.weekday - 1));
  final DateTime startDate;
  final int totalWeeks;
}