第一种: "compare:"

 // 时间字符串
    NSString *createdAtString = @"2015-11-20 11:10:05";
    NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
    fmt.dateFormat = @"yyyy-MM-dd HH:mm:ss";

    NSDate *createdAtDate = [fmt dateFromString:createdAtString];

    // 手机当前时间
    NSDate *nowDate = [NSDate date];

    /**
     NSComparisonResult的取值
     NSOrderedAscending = -1L, // 升序, 越往右边越大
     NSOrderedSame,  // 相等
     NSOrderedDescending // 降序, 越往右边越小
     */
    // 获得比较结果(谁大谁小)
    NSComparisonResult result = [nowDate compare:createdAtDate];

    if (result == NSOrderedAscending) { // 升序, 越往右边越大
        NSLog(@"createdAtDate > nowDate");
    } else if (result == NSOrderedDescending) { // 降序, 越往右边越小
        NSLog(@"createdAtDate < nowDate");
    } else {
        NSLog(@"createdAtDate == nowDate");
    }

第二种:"timeIntervalSince......"

// 时间字符串
    NSString *createdAtString = @"2015-11-20 09:10:05";
    NSDateFormatter *fmt = [[NSDateFormatter alloc] init];
    fmt.dateFormat = @"yyyy-MM-dd HH:mm:ss";
    NSDate *createdAtDate = [fmt dateFromString:createdAtString];

    // 手机当前时间
    //    NSDate *nowDate = [NSDate date];

    // 获得createdAtDate和nowDate的时间间隔(间隔多少秒)
    
    //    NSTimeInterval interval = [nowDate timeIntervalSinceDate:createdAtDate];
    NSTimeInterval interval = [createdAtDate timeIntervalSinceNow];
    
    NSLog(@"%f", interval);

第三种:"components:............................."

NSDateFormatter * fmt = [[NSDateFormatter alloc] init];
fmt.dateFormat = @"yyyy-MM-dd HH:mm:ss";
// 时间字符串
NSString * createdAtString = @"2016-11-01 09:10:05";
NSDate * createdAtDate = [fmt dateFromString: createdAtString];

// 当前时间
NSDate * nowDate = [NSDate date];

// 获得NSCalendar
NSCalendar * calendar = nil;
if ([NSCalendar respondsToSelector:@selector(calendarWithIdentifier:)]) {
    calendar = [NSCalendar calendarWithIdentifier:NSCalendarIdentifierGregorian];
} else {
calendar = [NSCalendar currentCalendar];
}

NSCalendarUnit unit = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;

NSDateComponents * cmps = [calendar components:unit fromDate:createdAtDate toDate:nowDate options:0];

NSLog(@"%@",cmps);