Как изменить программу?

#include <iostream>
using namespace std;
 
class DigitalTime
{
public:
           
    friend bool operator ==(const DigitalTime& time1, const DigitalTime& time2);
    //Returns true if time1 and time2 represent the same time; 
    //otherwise, returns false.
 
    DigitalTime(int the_hour, int the_minute);
    //Precondition: 0 <= the_hour <= 23 and 0 <= the_minute <= 59.
    //Initializes the time value to the_hour and the_minute.
 
    DigitalTime( );
    
    void interval_since(const DigitalTime& previous, int& hours,
         int& minutes) const;
    
 
    void advance(int minutes_added);
    
 
    void advance(int hours_added, int minutes_added);
    
 
    friend istream& operator >>(istream& ins, DigitalTime& the_object);
    
 
    friend ostream& operator <<(ostream& outs, const DigitalTime& the_object);
    
private:
    int hour;
    int minute;
 
};

#include <iostream>
#include <cctype>
#include <cstdlib>
#include "DigitalTime.h"
using namespace std;
 
 
 
void read_hour(istream& ins, int& the_hour);
 
 
void read_minute(istream& ins, int& the_minute);
 
int digit_to_int(char c);
 
bool operator ==(const DigitalTime& time1, const DigitalTime& time2)
{
    return (time1.hour == time2.hour && time1.minute == time2.minute);
}
 
//Uses iostream and cstdlib:
DigitalTime::DigitalTime(int the_hour, int the_minute)
{
    if (the_hour < 0 || the_hour > 23 || the_minute < 0 || the_minute > 59)
    {
        cout << "Illegal argument to DigitalTime constructor.";
        exit(1);
    }
    else
    {
        hour = the_hour;
        minute = the_minute;
    }
}
 
DigitalTime::DigitalTime( ) : hour(0), minute(0)
{
    //Body intentionally empty.
}
 
void DigitalTime::advance(int minutes_added)
{
    int gross_minutes = minute + minutes_added;
    minute = gross_minutes%60;
 
    int hour_adjustment = gross_minutes/60;
    hour = (hour + hour_adjustment)%24;
}
 
void DigitalTime::advance(int hours_added, int minutes_added)
{
    hour = (hour + hours_added)%24;
    advance(minutes_added);
}
 
 
void DigitalTime::interval_since(const DigitalTime& previous, int& hours,
     int& minutes) const
{
 
    hours = hour - previous.hour;
    if ((hours < 0) || (hours == 0 && (minute < previous.minute)))
        hours = hours + 24;
    minutes = minute - previous.minute;
 
    if (minutes < 0)
    {
        hours = hours - 1;
        minutes = minutes + 60;
    }  
}
 
 
//Uses iostream:
ostream& operator <<(ostream& outs, const DigitalTime& the_object)
{
    outs << the_object.hour << ':';
    if (the_object.minute < 10)
        outs << '0';
    outs << the_object.minute;
    return outs;
}
 
//Uses iostream:
istream& operator >>(istream& ins, DigitalTime& the_object)
{
    read_hour(ins, the_object.hour);
    read_minute(ins, the_object.minute);
    return ins;
}
 
int digit_to_int(char c)
{
    return ( int(c) - int('0') );
}
 
//Uses iostream, cctype, and cstdlib:
void read_minute(istream& ins, int& the_minute)
{
    char c1, c2;
    ins >> c1 >> c2;
 
    if (!(isdigit(c1) && isdigit(c2)))
    {
        cout << "Error illegal input to read_minute\n";
        exit(1);
    }
 
    the_minute = digit_to_int(c1)*10 + digit_to_int(c2);
 
    if (the_minute < 0 || the_minute > 59)
    {
        cout << "Error illegal input to read_minute\n";
        exit(1);
    }
}
 
//Uses iostream, cctype, and cstdlib:
void read_hour(istream& ins, int& the_hour)
{
    char c1, c2;
    ins >> c1 >> c2;
    if ( !( isdigit(c1) && (isdigit(c2) || c2 == ':' ) ) )
    {
        cout << "Error illegal input to read_hour\n";
        exit(1);
    }
 
    if (isdigit(c1) && c2 == ':')
    {
        the_hour = digit_to_int(c1);
    }
    else //(isdigit(c1) && isdigit(c2))
    {
        the_hour = digit_to_int(c1)*10 + digit_to_int(c2);
        ins >> c2;//discard ':'
        if (c2 != ':')
        {
                cout << "Error illegal input to read_hour\n";
                exit(1);
        }
    }
 
    if ( the_hour < 0 || the_hour > 23 )
    {
        cout << "Error illegal input to read_hour\n";
        exit(1);
    }
}