NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm +0800"];
NSString *destDateString = [dateFormatter stringFromDate:selected];
格式化時間……

在學習橘子的那本書，看了jxdwinter和 celvinren的回答後寫了下面的程式碼，顯示正常，多謝了！
附上程式碼

NSDate *selected = [datePicker date];
    NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
    [dateFormat setDateFormat:@"yyyy-MM-dd HH:mm"];
    NSString *theDate = [dateFormat stringFromDate:selected];
    
    NSString *message = [[NSString alloc] initWithFormat:
                         @"The date and time you selected is:%@",theDate];