用c来解决`NSDateFormatter`来处理很多日期格式

#define NSTimeIntervalSince1970  978307200.0

- (NSTimeInterval)timeIntervalSince1970
{
   
    return NSTimeIntervalSince1970;
}

+ (NSDate *)dateFromISO8601String:(NSString *)string {
    if (!string) {
        return nil;
    }
    
    struct tm tm;
    time_t t;
    
    strptime([string cStringUsingEncoding:NSUTF8StringEncoding], "%Y-%m-%dT%H:%M:%S%z", &tm);
    tm.tm_isdst = -1;
    t = mktime(&tm);
    
    return [NSDate dateWithTimeIntervalSince1970:t + [[NSTimeZone localTimeZone] secondsFromGMT]];
}


- (NSString *)ISO8601String
{
    struct tm *timeinfo;
    char buffer[80];
    
    time_t rawtime = [self timeIntervalSince1970] - [[NSTimeZone localTimeZone] secondsFromGMT];
    timeinfo = localtime(&rawtime);
    
    strftime(buffer, 80, "%Y-%m-%dT%H:%M:%S%z", timeinfo);
    
    return [NSString stringWithCString:buffer encoding:NSUTF8StringEncoding];
}