有一个题需要解答解惑

构造一个日期时间类Timedate，实现对日期（年月日）时间（时分秒）的设置及输出

#include <iostream>
#include <iomanip>
using namespace std;
/**********Program**********/
class Timedate
{
public:
    Timedate(int syear = 2000, int smonth = 1, int sdate = 1)
    { //构造函数完成所有私有成员的初始化
        year = syear;
        month = smonth;
        date = sdate;
        hh = 00;
        mm = 00;
        ss = 00;
    }

    void list(); //完成判断
    void putdate(int syear, int smonth, int sdate)
    { //完成年月日的设置
        year = syear;
        month = smonth;
        date = sdate;
    }
    void puttime(int hour, int minute, int second) //内联函数：完成时分秒的设置
    {
        hh = hour;
        mm = minute;
        ss = second;
    }

private:
    int year, month, date, hh, mm, ss;
};
/**********  End  **********/
void Timedate::list()
{
    cout << "year/month/date :";
    switch (year)
    {
    case 2000:
        cout << "2000";
        break;
    case 2001:
        cout << "2001";
        break;
    case 2002:
        cout << "2002";
        break;
    case 2003:
        cout << "2003";
        break;
    case 2004:
        cout << "2004";
        break;
    case 2005:
        cout << "2005";
        break;
    }
    switch (month)
    {
    case 1:
        cout << '/' << "Jan";
        break;
    case 2:
        cout << '/' << "Feb";
        break;
    case 3:
        cout << '/' << "Mar";
        break;
    case 4:
        cout << '/' << "Apr";
        break;
    case 5:
        cout << '/' << "May";
        break;
    case 6:
        cout << '/' << "Jun";
        break;
    case 7:
        cout << '/' << "Jul";
        break;
    case 8:
        cout << '/' << "Aug";
        break;
    case 9:
        cout << '/' << "Sep";
        break;
    case 10:
        cout << '/' << "Oct";
        break;
    case 11:
        cout << '/' << "Nov";
        break;
    case 12:
        cout << '/' << "Dec";
        break;
    }
    cout << '/' << date << endl;
    cout << "hour:minite:second :";
    cout << hh << ':' << mm << ':' << ss << endl;
}

int main()
{
    Timedate A(2004, 3, 3), B;
    A.list();
    B.list();
    B.putdate(2005, 10, 18);
    B.puttime(17, 30, 10);
    B.list();
    return 0;
}

#include <iostream>
#include <ctime>
#include <iomanip>

class Timedate
{
public:
    Timedate()
    {
        _time = std::time(nullptr);
    }

    Timedate(int year, int month, int day, int hour, int minute, int second)
    {
        std::tm tm{};
        tm.tm_year = year - 1900;
        tm.tm_mon = month - 1;
        tm.tm_mday = day;
        tm.tm_hour = hour;
        tm.tm_min = minute;
        tm.tm_sec = second;
        _time = std::mktime(&tm);
    }

    int year() const
    {
        std::tm *tm = std::localtime(&_time);
        return tm->tm_year + 1900;
    }

    void setYear(int year)
    {
        std::tm *tm = std::localtime(&_time);
        tm->tm_year = year - 1900;
        _time = std::mktime(tm);
    }

    int month() const
    {
        std::tm *tm = std::localtime(&_time);
        return tm->tm_mon + 1;
    }

    void setMonth(int month)
    {
        std::tm *tm = std::localtime(&_time);
        tm->tm_mon = month - 1;
        _time = std::mktime(tm);
    }

    int day() const
    {
        std::tm *tm = std::localtime(&_time);
        return tm->tm_mday;
    }

    void setDay(int day)
    {
        std::tm *tm = std::localtime(&_time);
        tm->tm_mday = day;
        _time = std::mktime(tm);
    }

    int hour() const
    {
        std::tm *tm = std::localtime(&_time);
        return tm->tm_hour;
    }

    void setHour(int hour)
    {
        std::tm *tm = std::localtime(&_time);
        tm->tm_hour = hour;
        _time = std::mktime(tm);
    }

    int minute() const
    {
        std::tm *tm = std::localtime(&_time);
        return tm->tm_min;
    }

    void setMinute(int minute)
    {
        std::tm *tm = std::localtime(&_time);
        tm->tm_min = minute;
        _time = std::mktime(tm);
    }

    int second() const
    {
        std::tm *tm = std::localtime(&_time);
        return tm->tm_sec;
    }

    void setSecond(int second)
    {
        std::tm *tm = std::localtime(&_time);
        tm->tm_sec = second;
        _time = std::mktime(tm);
    }

private:
    std::time_t _time;
};

std::ostream &operator<<(std::ostream &os, const Timedate &timedate)
{
    std::cout << timedate.year() << '-' << timedate.month() << '-' << timedate.day() << ' '
              << std::setw(2) << std::setfill('0') << timedate.hour() << ':'
              << std::setw(2) << std::setfill('0') << timedate.minute() << ':'
              << std::setw(2) << std::setfill('0') << timedate.second();
    return os;
}

int main()
{
    Timedate currentDateTime;
    std::cout << "当前日期和时间是: " << currentDateTime << std::endl;
    int year, month, day, hour, minute, second;
    char ch;
    std::cout << "输入日期和时间(格式: YYYY-MM-DD HH:MM::SS): ";
    std::cin >> year >> ch >> month >> ch >> day >> hour >> ch >> minute >> ch >> second;
    Timedate timedate(year, month, day, hour, minute, second);
    std::cout << "输出: " << timedate << std::endl;
    return 0;
}