可以寫一個加號方法，放在類別裡面
這裡我寫的是一個減號方法，直接在類裡面呼叫

-(NSString*)weekdayStringFromDate:(NSDate*)inputDate {

    NSArray *weekdays = [NSArray arrayWithObjects: [NSNull null], @"星期天", @"星期一", @"星期二", @"星期三", @"星期四", @"星期五", @"星期六", nil];

    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

    NSTimeZone *timeZone = [[NSTimeZone alloc] initWithName:@"Asia/SuZhou"
 
];

    [calendar setTimeZone: timeZone];

    NSCalendarUnit calendarUnit = NSWeekdayCalendarUnit;

    NSDateComponents *theComponents = [calendar components:calendarUnit fromDate:inputDate];

    return [weekdays objectAtIndex:theComponents.weekday];

}