//Header file dtime.h

#include <iostream>
using namespace std;

class DigitalTime
{
public:
	friend bool operator ==(const DigitalTime& time1, const DigitalTime& time2);

	DigitalTime(int the_hour, int the_minute);
	DigitalTime();
	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;
};
//End of dtime.h

#include <iostream>
#include <cctype>
#include <cstdlib>
#include "dtime.h"
using namespace std;

//These function declarations are for use in the defintion of the overloaded
//input operator >>:

void read_hour(istream& ins, int& the_hour);
//Precondition: Next input in the stream ins is a time in 24 hour notation.
//Postcondition: the_hour has been set to the hour part of the time.
//THe colon has been discarded and the next input to be read is the minute.

void read_minute(istream& ins, int& the_minute);
//reads the minute from the stream ins after read_hour has read the hour.

int digit_to_int(char c);
//Precondition: c is one of the digits '0' thru '9'.
//returns the integer for the digit.


//friend function.
bool operator ==(const DigitalTime& time1, const DigitalTime& time2)
{
	return (time1.hour == time2.hour && time1.minute == time2.minute);
}

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 and minute are private global variables.
		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);
}

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;
}

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 (static_cast<int>(c) - static_cast<int>('0'));
}

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);
	}

}

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);
	}
	else
	{
		the_hour = digit_to_int(c1)*10 + digit_to_int(c2);
		ins >> c2;
		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);
	}
}
//end of dtime.cpp

dtimedemo.cpp

#include <iostream>
#include "dtime.h"
using namespace std;

int main()
{
	DigitalTime clock, old_clock;

	cout << "Enter the time in 24-hour notation: ";
	cin >> clock;

	old_clock = clock;
	clock.advance(15);
	if (clock == old_clock)
		cout << "Something is wrong.";
	cout << "You entered " << old_clock << 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;
}
//end of dtimedemo.cpp

Error message:

/tmp/cc1qikgM.o: In function `main':dtimedemo.cpp:(.text+0x8a): undefined reference to `DigitalTime::DigitalTime()'
:dtimedemo.cpp:(.text+0x95): undefined reference to `DigitalTime::DigitalTime()'
:dtimedemo.cpp:(.text+0xbc): undefined reference to `operator>>(std::basic_istream<char, std::char_traits<char> >&, DigitalTime&)'
:dtimedemo.cpp:(.text+0xdb): undefined reference to `DigitalTime::advance(int)'
:dtimedemo.cpp:(.text+0xed): undefined reference to `operator==(DigitalTime const&, DigitalTime const&)'
:dtimedemo.cpp:(.text+0x12a): undefined reference to `operator<<(std::basic_ostream<char, std::char_traits<char> >&, DigitalTime const&)'
:dtimedemo.cpp:(.text+0x15f): undefined reference to `operator<<(std::basic_ostream<char, std::char_traits<char> >&, DigitalTime const&)'
:dtimedemo.cpp:(.text+0x18a): undefined reference to `DigitalTime::advance(int, int)'
:dtimedemo.cpp:(.text+0x1bf): undefined reference to `operator<<(std::basic_ostream<char, std::char_traits<char> >&, DigitalTime const&)'

END OF ERROR MESSAGE

g++ -o dtime.o dtime.cpp
g++ -o dtimedemo dtimedemo.cpp dtime.o

g++ -o dtimedemo dtimedemo.cpp dtime.cpp