在开发的过程中，会遇到一些关于时间的处理，在这里总结一下。
一、获取当前时间

+(NSString *)getCurrentTime{
    //获取当前时间，日期
    NSDate *currentDate = [NSDate date];
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"yyyy-MM"];//此处可以改成你需要的时间格式
    NSString *currentDateString = [dateFormatter stringFromDate:currentDate];
    return currentDateString;
}

二、获取当前时间戳

-(NSString *)getNowTimeTimestamp{

    NSDate *dat = [NSDate dateWithTimeIntervalSinceNow:0];
    NSTimeInterval a=[dat timeIntervalSince1970];
    NSLog(@"时间==%d",(int)a);
    NSString*timeString = [NSString stringWithFormat:@"%0.f", a];//转为字符型;
    return timeString;
}

三、将UTC日期字符串转为本地时间字符串
-后台返回的时间格式-2018-06-13T15:26:31Z

+(NSString *)getLocalDateFormateUTCDate:(NSString *)utcDate {
    
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    //输入格式
    [dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ssZ"];
    NSTimeZone *localTimeZone = [NSTimeZone localTimeZone];
    [dateFormatter setTimeZone:localTimeZone];
    NSDate *dateFormatted = [dateFormatter dateFromString:utcDate];
    //输出格式
    [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSString *dateString = [dateFormatter stringFromDate:dateFormatted];
    return dateString;
}

四、将本地日期字符串转为UTC日期字符串
本地日期格式:2013-08-03 12:53:51

+ (NSString *)getUTCFormateLocalDate:(NSString *)localDate {
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    //输入格式
    [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSDate *dateFormatted = [dateFormatter dateFromString:localDate];
    NSTimeZone *timeZone = [NSTimeZone timeZoneWithName:@"UTC"];
    [dateFormatter setTimeZone:timeZone];
    //输出格式
    [dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ssZ"];
    NSString *dateString = [dateFormatter stringFromDate:dateFormatted];
    return dateString;
}

五、
时间解析:后台给的格式为yyyy-MM-dd HH:mm:ss.SSS
转换成XXX之前

+ (NSString *)compareCurrentTime:(NSString *)str
{
    
    //把字符串转为NSdate
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss.SSS"];
    NSDate *timeDate = [dateFormatter dateFromString:str];
    
    //得到与当前时间差
    NSTimeInterval  timeInterval = [timeDate timeIntervalSinceNow];
    timeInterval = -timeInterval;
    //标准时间和北京时间差8个小时
    timeInterval = timeInterval - 8*60*60;
    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;
}

六、时间解析:后台给的为时间戳
转换成XXX之前

+(NSString *)timeFromTimestamp:(NSInteger)timestamp{
    
    NSDateFormatter *dateFormtter =[[NSDateFormatter alloc] init];
    NSDate *d = [NSDate dateWithTimeIntervalSince1970:timestamp];
    NSTimeInterval late=[d timeIntervalSince1970]*1;    //转记录的时间戳
    NSDate* dat = [NSDate dateWithTimeIntervalSinceNow:0];
    NSTimeInterval now=[dat timeIntervalSince1970]*1;   //获取当前时间戳
    NSString *timeString=@"";
    NSTimeInterval cha=now-late;
    // 发表在一小时之内
    if (cha/3600<1) {
        if (cha/60<1) {
            timeString = @"1";
        }
        else
        {
            timeString = [NSString stringWithFormat:@"%f", cha/60];
            timeString = [timeString substringToIndex:timeString.length-7];
        }
        
        timeString=[NSString stringWithFormat:@"%@分钟前", timeString];
    }
    // 在一小时以上24小以内
    else if (cha/3600>1&&cha/86400<1) {
        timeString = [NSString stringWithFormat:@"%f", cha/3600];
        timeString = [timeString substringToIndex:timeString.length-7];
        timeString=[NSString stringWithFormat:@"%@小时前", timeString];
    }
    // 发表在24以上10天以内
    else if (cha/86400>1&&cha/86400*3<1)     //86400 = 60(分)*60(秒)*24(小时)   3天内
    {
        timeString = [NSString stringWithFormat:@"%f", cha/86400];
        timeString = [timeString substringToIndex:timeString.length-7];
        timeString=[NSString stringWithFormat:@"%@天前", timeString];
    }
    // 发表时间大于10天
    else
    {
        [dateFormtter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
        timeString = [dateFormtter stringFromDate:d];
    }
    
    return timeString;
}

七、判读是不是今天以前的日期

+(BOOL)isTodayBefore:(NSString *)year month:(NSString *)month day:(NSString *)day{
    
    //选择的结束日期
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:@"YYYY-MM-dd HH:mm:ss"];
    NSString* beginStr = [NSString stringWithFormat:@"%@-%@-%@ 00:00:00",year,month,day];
    NSDate *beginDate=[formatter dateFromString:beginStr];
    NSTimeInterval endTime= [beginDate timeIntervalSince1970]; //转换生时间戳
    
    //今天的时间日期
    NSLog(@"转换成时间搓====%f",endTime);
    NSDate *datenow = [NSDate date];
    NSTimeInterval nowTime = [datenow timeIntervalSince1970];
    
    if (endTime>nowTime) {
        
        return YES;
    }else{
        return NO;
    }
    
}