在初始化NSDateFormatter时，需要添加上下面3句：

NSLocale *zh_CNLocale = [[NSLocale alloc] initWithLocaleIdentifier:@"zh_CN"];
dateFormatter.locale =zh_CNLocale;
dateFormatter.calendar = [[NSCalendar alloc]initWithCalendarIdentifier:NSCalendarIdentifierISO8601];

swift版

dateFormatter.locale = Locale(identifier: "zh_CN")
dateFormatter.calendar = Calendar(identifier: .iso8601)

例1：日期字符串转Date

+ (NSDate *)dateWithString:(NSString *)dateStr {
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];

    NSLocale *zh_CNLocale = [[NSLocale alloc] initWithLocaleIdentifier:@"zh_CN"];
    dateFormatter.locale =zh_CNLocale;
    dateFormatter.calendar = [[NSCalendar alloc]initWithCalendarIdentifier:NSCalendarIdentifierISO8601];
    
    NSDate *birthdayDate = [dateFormatter dateFromString:dateStr];
    return birthdayDate;
}

例2：日期字符串转时间戳

+ (long)timeIntervalFromDataStr:(NSString *)dateStr format:(NSString *)format {
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:format];
    NSLocale *zh_CNLocale = [[NSLocale alloc] initWithLocaleIdentifier:@"zh_CN"];
    dateFormatter.locale =zh_CNLocale;
    dateFormatter.calendar = [[NSCalendar alloc]initWithCalendarIdentifier:NSCalendarIdentifierISO8601];
    
    NSDate *date = [dateFormatter dateFromString:dateStr];
    NSInteger timeInterval = [date timeIntervalSince1970] * 1000;
    return timeInterval;
}