1.獲取當前時間的年月日時分秒

    // 獲取當前時間
    NSDate *now = [NSDate date];
    NSLog(@"now = %@", now);
    // 日歷
    NSCalendar *calendar1 = [NSCalendar currentCalendar];
    // 利用日歷類從當前時間對象中獲取 年月日時分秒(單獨獲取出來)
    // components: 參數的含義是, 問你需要獲取什么?
    // 一般情況下如果一個方法接收一個參數, 這個參數是是一個枚舉 , 那么可以通過|符號, 連接多個枚舉值
    NSCalendarUnit type = NSCalendarUnitYear |
                          NSCalendarUnitMonth |
                          NSCalendarUnitDay |
                          NSCalendarUnitHour |
                         NSCalendarUnitMinute |
                        NSCalendarUnitSecond;
    NSDateComponents *cmps = [calendar1 components:type fromDate:now];
    NSLog(@"year = %ld", cmps.year);
    NSLog(@"month = %ld", cmps.month);
    NSLog(@"day = %ld", cmps.day);
    NSLog(@"hour = %ld", cmps.hour);
    NSLog(@"minute = %ld", cmps.minute);
    NSLog(@"second = %ld", cmps.second);

2.比較兩個時間之間的差值, 比較相差多少年多少月多少日多少小時多少分鐘多少秒

    // 2.1過去的一個時間
    NSString *str = @"2015-06-29 07:05:26 +0000";
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    formatter.dateFormat = @"yyyy-MM-dd HH:mm:ss Z";
    NSDate *date = [formatter dateFromString:str];
    // 2.2當前的時間
    NSDate *now = [NSDate date];
    
    NSLog(@"date = %@", date);
    NSLog(@"now = %@", now);
    
    // 2.3比較兩個時間
    NSCalendar *calendar = [NSCalendar currentCalendar];
    NSCalendarUnit type = NSCalendarUnitYear |
    NSCalendarUnitMonth |
    NSCalendarUnitDay |
    NSCalendarUnitHour |
    NSCalendarUnitMinute |
    NSCalendarUnitSecond;
    NSDateComponents *cmps = [calendar components:type fromDate:date toDate:now options:0];
    NSLog(@"%ld年%ld月%ld日%ld小時%ld分鐘%ld秒鐘", cmps.year, cmps.month, cmps.day, cmps.hour, cmps.minute, cmps.second);