头文件程序：
#ifndef DTIME_H
#define DTIME_H
#include<iostream>
using namespace std;
class DigitalTime
{
public:
    DigitalTime(int theHour,int theMinute);
    DigitalTime();
    int getHour() const;
    int getMinute() const;
    void advance(int minutesAdded);
    void advance(int hoursAdded,int minutesAdded);
    friend bool operator==(const DigitalTime& time1,
                            const DigitalTime& time2);
    friend istream operator>>(istream& ins,DigitalTime& theObject);
    friend ostream operator<<(ostream& outs,const DigitalTime& theObject);
private:
    int hour;
    int minute;
    static void readHour(int& theHour);
    static void readMinute(int& theMinute);
    static int digitToInt(char c);
};
#endif // DTIME_H
函数程序：
#include "dtime.h"
#include<iostream>
#include<cctype>
#include<cstdlib>
using namespace std;
DigitalTime::DigitalTime(int theHour, int theMinute)
{
    if(theHour<0||theHour>24||theMinute<0||theMinute>59)
    {
        cout<<"Illegal argument to DigitalTime constructor.";
        exit(1);
    }
    else
    {
        hour=theHour;
        minute=theMinute;
    }
    if(hour==24)
    {
        hour=0;
        minute=0;
    }
}
DigitalTime::DigitalTime()
{
    hour=0;
    minute=0;
}
int DigitalTime::getHour()const
{
    return hour;
}
int DigitalTime::getMinute() const
{
    return minute;
}
void DigitalTime::advance(int minutesAdded)
{
    int grossMinutes=minute+minutesAdded;
    minute=grossMinutes%60;
    int hourAdjustment=grossMinutes/60;
    hour=(hour+hourAdjustment)%24;
}
void DigitalTime::advance(int hoursAdded, int minutesAdded)
{
    hour=(hour+hoursAdded)%24;
    advance(minutesAdded);
}
bool operator ==(const DigitalTime& time1,const DigitalTime& time2)
{
    return (time1.hour==time2.hour&&time1.minute==time2.minute);
}
istream& operator <<(istream& ins,DigitalTime& theObject)
{
    DigitalTime::readHour(theObject.hour);
    DigitalTime::readMinute(theObject.minute);
    return ins;
}
ostream& operator >>(ostream& outs,DigitalTime& theObject)
{
    outs<<theObject.hour<<":";
    if(theObject.minute<10)
        outs<<'0';
    outs<<theObject.minute;
    return outs;
}
int DigitalTime::digitToInt(char c)
{
    return (static_cast<int>(c)-static_cast<int>('0'));
}
void DigitalTime::readMinute(int &theMinute)
{
    char c1,c2;
    cin>>c1>>c2;
    if(!(isdigit(c1)&&isdigit(c2)))
    {
        cout<<"Error:illegal input to readMinute\n";
        exit(1);
    }
    theMinute=digitToInt(c1)*10+digitToInt(c2);
    if(theMinute<0||theMinute>59)
    {
        cout<<"Error:illegal input to readMinute\n";
        exit(1);
    }
}
void DigitalTime::readHour(int &theHour)
{
    char c1,c2;
    cin>>c1>>c2;
    if(!(isdigit(c1)&&(isdigit(c2)||c2==':')))
    {
        cout<<"Error:illegal input to readHour\n";
        exit(1);
    }
    if(isdigit(c1)&&c2==':')
    {
        theHour=DigitalTime::digitToInt(c1);
    }
    else
    {
        theHour=DigitalTime::digitToInt(c1)*10
                +DigitalTime::digitToInt(c2);
        cin>>c2;
        if(c2!=':')
        {
            cout<<"Error: illegal input to readHour\n";
            exit(1);
        }
    }
    if(theHour==24)
        theHour=0;
    if(theHour<0||theHour>23)
    {
        cout<<"Error: illegal input to readHour\n";
        exit(1);
    }
}
主函数程序：
#include<iostream>
using namespace std;
#include"dtime.h"
int main()
{
    DigitalTime clock,oldClock;
    cout<<"You may write midnight as either 0:00 or 24:00,\n"
       <<"but I will always write it as 0:00.\n"
      <<"Enter the time in 24-hour notation:";
    cin>>clock;
    oldClock=clock;
    clock.advance(15);
    if(clock==oldClock)
        cout<<"Something is wrong.";
    cout<<"You entered "<<oldClock<<endl;
    cout<<"15 minutes later the time will be"<<clock<<endl;
    clock.advance(2,15);
    cout<<"2 hours and 15 minutes after that\n"
       <<"the time will be "
      <<clock<<endl;
    return 0;
}


程序中istream和ostream函数部分有错误应该怎么改

多谢大神