起始本人使用的是以下格式输出时间显示的

- (NSString *) compareCurrentTime:(NSString *)str
{
    //把字符串转为NSdate
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    NSTimeZone* timeZone = [NSTimeZone timeZoneWithName:@"Asia/Beijing"];
    [dateFormatter setTimeZone:timeZone];
    [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSDate *timeDate = [dateFormatter dateFromString:str];
    
    //得到与当前时间差
    NSTimeInterval  timeInterval = [timeDate timeIntervalSinceNow];
    timeInterval = -timeInterval;
    long temp = 0;
    NSString *result;
    if (timeInterval < 60) {
        result = [NSString stringWithFormat:@"刚刚"];
    }
    else if((temp = timeInterval/60) <60){
        result = [NSString stringWithFormat:@"%ld分钟前",temp];
    }
    else if((temp = temp/60) <24){
        result = [NSString stringWithFormat:@"%ld小时前",temp];
    }
    else if((temp = temp/24) <30){
        result = [NSString stringWithFormat:@"%ld天前",temp];
    }
    else if((temp = temp/30) <12){
        result = [NSString stringWithFormat:@"%ld月前",temp];
    }
    else{
        temp = temp/12;
        result = [NSString stringWithFormat:@"%ld年前",temp];
    }
    return  result;
}

之后查找资料后发现一种更好的时间格式显示效果，便记录于此。原文于此

- (NSString *)format:(NSString *)string{
    NSDateFormatter *inputFormatter= [[NSDateFormatter alloc] init];
    [inputFormatter setLocale:[[NSLocale alloc] initWithLocaleIdentifier:@"en_US"] ];
    [inputFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSDate*inputDate = [inputFormatter dateFromString:string];
    //NSLog(@"startDate= %@", inputDate);
    
    NSDateFormatter *outputFormatter= [[NSDateFormatter alloc] init];
    [outputFormatter setLocale:[NSLocale currentLocale]];
    [outputFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    //get date str
    NSString *str= [outputFormatter stringFromDate:inputDate];
    //str to nsdate
    NSDate *strDate = [outputFormatter dateFromString:str];
    //修正8小时的差时
    NSTimeZone *zone = [NSTimeZone systemTimeZone];
    NSInteger interval = [zone secondsFromGMTForDate: strDate];
    NSDate *endDate = [strDate  dateByAddingTimeInterval: interval];
    //NSLog(@"endDate:%@",endDate);
    NSString *lastTime = [self compareDate:endDate];
    NSLog(@"lastTime = %@",lastTime);
    return lastTime;
}

-(NSString *)compareDate:(NSDate *)date{
    
    NSTimeInterval secondsPerDay = 24 * 60 * 60;
    
    //修正8小时之差
    NSDate *date1 = [NSDate date];
    NSTimeZone *zone = [NSTimeZone systemTimeZone];
    NSInteger interval = [zone secondsFromGMTForDate: date1];
    NSDate *localeDate = [date1  dateByAddingTimeInterval: interval];
    
    //NSLog(@"nowdate=%@\nolddate = %@",localeDate,date);
    NSDate *today = localeDate;
    NSDate *yesterday,*beforeOfYesterday;
    //今年
    NSString *toYears;
    
    toYears = [[today description] substringToIndex:4];
    
    yesterday = [today dateByAddingTimeInterval: -secondsPerDay];
    beforeOfYesterday = [yesterday dateByAddingTimeInterval: -secondsPerDay];
    
    // 10 first characters of description is the calendar date:
    NSString *todayString = [[today description] substringToIndex:10];
    NSString *yesterdayString = [[yesterday description] substringToIndex:10];
    NSString *beforeOfYesterdayString = [[beforeOfYesterday description] substringToIndex:10];
    
    NSString *dateString = [[date description] substringToIndex:10];
    NSString *dateYears = [[date description] substringToIndex:4];
    
    NSString *dateContent;
    if ([dateYears isEqualToString:toYears]) {//同一年
        //今 昨 前天的时间
        NSString *time = [[date description] substringWithRange:(NSRange){11,5}];
        //其他时间
        NSString *time2 = [[date description] substringWithRange:(NSRange){5,11}];
        if ([dateString isEqualToString:todayString]){
            dateContent = [NSString stringWithFormat:@"今天 %@",time];
            return dateContent;
        } else if ([dateString isEqualToString:yesterdayString]){
            dateContent = [NSString stringWithFormat:@"昨天 %@",time];
            return dateContent;
        }else if ([dateString isEqualToString:beforeOfYesterdayString]){
            dateContent = [NSString stringWithFormat:@"前天 %@",time];
            return dateContent;
        }else{
            return time2;
        }
    }else{
        return dateString;
    }
}