日期计算

//Date.cpp
#include<iostream>
#include<time.h>
#include<cmath>
#include<cstring>
using namespace std;
//Date_class
class Date
{
private:
    int year, month, day;    
    int week;            //存放星期 
    void IncDay();       //日期增加一天函数 
    int distance();        //设成私有成员,由公有函数调用,距基准日期的天数,设基准日期为1900-1-1---B
    int Week() {         //计算星期
        return week;
    }           
public:
    Date(int y = 1900,int m = 1,int d = 1);   //构造函数(default)
    void SetSystemDate();         //取得系统日期 
    void SetDate(int yy,int mm,int dd);       //日期设置

    bool IsLeapYear();                        //是否闰年

    void print_ymd() const;                   //年月日输出法 
    void print_mdy() const;                    //月日年输出法

    void AddDay(int days);                    //A:增加任意天数
    bool IsEndofMonth() ;                     //判断是否到了月末--A
    int Daysof2Date(Date ymd) ;               //B:2个日期之间的天数---区别查表法 

    void ShowWeek(ostream &out);              //显示星期 
    int operator - (Date ymd);                //重载 —运算符 
    friend ostream& operator << (ostream &out, Date& ymd);   //输出日期 
};

Date::Date(int y,int m,int d){
    SetDate(y,m,d);
} 

void Date::SetSystemDate()         //取得系统日期 
{
    tm *gm;                     //tm 是时间(含年月日分秒的结构体) 
    time_t t = time(NULL);      //time_t 是长整型,,,t是总秒数;
    gm = gmtime(&t);            //函数返回给定的统一世界时间（通常是格林威治时间），如果系统不支持统一世界时间系统返回NULL 
    year = 1900 + gm->tm_year;     //现在的年月日分秒 
    month = gm->tm_mon + 1;
    day = gm->tm_mday;
}

void Date::SetDate(int yy,int mm,int dd) {    //附带合法性检查，错误返回系统当前日期 
    //进行日期合法性检查 
    if (yy >= 1900 && yy <= 9999)
        year = yy;
    else {
        cout << "年设置错误：1900~9999之间!(默认设置成系统当前日期)" << endl;
        SetSystemDate();
        return ;
    }
    if (mm >= 1 && mm <= 12)
        month = mm;
    else {
        cout << "月设置错误：1900~9999之间!(默认设置成系统当前日期)" << endl;
        SetSystemDate();
        return;
    }
    
    switch(month)
    {
        //小月天数
        case 4:
        case 6:
        case 9:
        case 11: 
        if (dd >= 1 && dd <= 30)
            day = dd;
        else {
                cout << "日设置错误：1900~9999之间!(默认设置成系统当前日期)" << endl;
                SetSystemDate();
                return ;
            }
        case 2:  //2月
            if ( (IsLeapYear() && dd >= 1 && dd <= 29) || (!IsLeapYear() && dd >= 1 && dd <= 28) )
                day = dd;
            else {
                cout << "日设置错误：1900~9999之间!(默认设置成系统当前日期)" << endl;
                SetSystemDate();
                return ;
            }
        default : //大月
            if (dd >= 1 && dd <= 31)
                day = dd;
            else {
                cout << "年设置错误：1900~9999之间!(默认设置成系统当前日期)" << endl;
                SetSystemDate();
                return ;
            }
    }
}

bool Date::IsLeapYear()
{
    return (year%4 == 0 && year%100 != 0) || (year%400 == 0);
}

void Date::print_ymd() const   //年月日输出法
{
    cout << year << "-" << month << "-" << day; 
}

void Date::print_mdy() const
{
    const char *MonthName[12] = {"Jau","Feb","Mar","Apri","May","June","July","Aug","Sept",
                                "Oct","Nov","Dec"};
    cout << MonthName[month-1] << " " << day << "," << year ; 
}

void Date::IncDay()             //增加一天
{
    if(IsEndofMonth())         //增加天数，记得要判断是否是年末，月末 
    {
        if(month == 12)
        {
            day = 1; month = 1; year++;
        }
        else               //已经是IsEndMonth 
            day = 1; month++;
    } 
    else            
        day++; 
}

bool Date::IsEndofMonth()
{
    switch(month)
    {
        case 4:
        case 6:
        case 9:
        case 11: return day == 30;
        case 2:
            if (IsLeapYear())
                return day == 29;
            else
                return day == 28;
        default:
            return day == 31;
    }
}
void Date::AddDay(int days)
{
    for (int i = 0; i <= days; i++)    //增加多少天--day1->day2 = day+1 
    {
        IncDay(); 
    }
}

/*int years = year - 1;    //相隔多少年，，用其算的才是前几年应该有的天数
    totalDays = years*365 + years/4 - years/100 + years/400 //因为DAY...是按闰年算的年份包含闰年的已经算好
        + DAYS_BEFORE_MONTH[month-1] + day;
    if (IsLeapYear() && month > 2) totalDays++;   
//(闰年)当大于2月时，天数要加1；(注：该法查表代码简洁，但是需要记住表)*/ 

int Date::distance()       //距离基准日期的天数, 先算到指定年的天数+一月到指定月份天数+指定天数 
{                         
    int days;
    int years = year-1900;
    days = years*365 + years/4 - years/100 + years/400;      //不计闰年的天数
//    if(years)                  // 
//        days += (years-1)/4;   //逢闰年+1天 
    switch(month)              //此处方法不错，从12月份算起,输入多少月份就能加到哪个月 
    {                           //注意，无break； 
        case 12: days = days + 30;
        case 11: days = days + 31;
        case 10: days = days + 30;
        case 9: days = days + 31;
        case 8: days = days + 31;
        case 7: days = days + 30;
        case 6: days = days + 31;
        case 5: days = days + 30;
        case 4: days = days + 31;
        case 3: 
            if (IsLeapYear())
                days = days + 29;
            else
                days = days + 28;
        case 2: days = days + 31;
        default : break; 
    } 
    days = days + day;         //多少天只要加多少天就可以了,(年月已经知道过了多少天)
    return days;
}

int Date::Daysof2Date(Date ymd) //两个日期的天数 
{
    int days;
    days = abs(distance()-ymd.distance()); //distance()返回值是指定日期到基准类的天数(本类日期-指定日期) 
    return days;               //2个日期之间的天数 
}

void Date::ShowWeek(ostream &out)  //显示该日期是星期几 
{
    week = distance() % 7;
    const char w[7][3] = {"天", "一", "二", "三", "四", "五", "六"};
    out << year << "-" << month << "-" << day << "--星期" << w[week] << endl; 
}

int Date::operator - (Date ymd)
{
    return Daysof2Date(ymd);
}

ostream& operator << (ostream& out, Date& ymd)
{
    ymd.ShowWeek(out);
    return out;
}

int main(void)
{
    Date date1, date2;
    int year, month, day; 
    cout << "please input year, month, day: " << endl;
    cin >> year >> month >> day;
//    date1.SetDate(2013,1,27);
//    cout << "the current date is : " << endl;
//    date1.print_ymd();
//    date1.print_mdy();
//    date1.AddDay(100);
//    cout << "After 100 days , the date is :";
//    date1.print_ymd();
    date1.SetDate(year, month, day);
    cout << date1;        
    
    cout << "please input the date(year,month,day) in the future : " << endl;
    cin >> year >> month >> day;
    date2.SetDate(year,month,day);
    date2.print_mdy();
    cout << " 距离 :";
    date1.print_mdy();
    cout << " have "<< date1-date2 << " days" << endl;     
    
    cout << "输入(想要经历的)天数(将显示当前日期后(N天)的日期: " << endl;
    cin >> day;
    date1.print_ymd();
    date1.AddDay(day);
    cout << ", After " << day << " days, the date is : " << date1;
    
    return 0; 
}