格式化新浪微博日期成NSDate

- (NSDate *)dateFromString:(NSString *)string {
    //Wed Mar 14 16:40:08 +0800 2012
    if (!string) return nil;
    struct tm tm;
    time_t t;
    string=[string substringFromIndex:4];
    strptime([string cStringUsingEncoding:NSUTF8StringEncoding], "%b %d %H:%M:%S %z %Y", &tm);
    tm.tm_isdst = -1;
    t = mktime(&tm);
    return [NSDate dateWithTimeIntervalSince1970:t];
}