how to code timer in c++ code example

Example: timer in c++

// CPP program to create a timer 
#include <iomanip> 
#include <iostream> 
#include <stdlib.h> 
#include <unistd.h> 
using namespace std; 

// hours, minutes, seconds of timer 
int hours = 0; 
int minutes = 0; 
int seconds = 0; 

// function to display the timer 
void displayClock() 
{ 
	// system call to clear the screen 
	system("clear"); 

	cout << setfill(' ') << setw(55) << "		 TIMER		 \n"; 
	cout << setfill(' ') << setw(55) << " --------------------------\n"; 
	cout << setfill(' ') << setw(29); 
	cout << "| " << setfill('0') << setw(2) << hours << " hrs | "; 
	cout << setfill('0') << setw(2) << minutes << " min | "; 
	cout << setfill('0') << setw(2) << seconds << " sec |" << endl; 
	cout << setfill(' ') << setw(55) << " --------------------------\n"; 
} 

void timer() 
{ 
	// infinte loop because timer will keep 
	// counting. To kill the process press 
	// Ctrl+D. If it does not work ask 
	// ubuntu for other ways. 
	while (true) { 
		
		// display the timer 
		displayClock(); 

		// sleep system call to sleep 
		// for 1 second 
		sleep(1); 

		// increment seconds 
		seconds++; 

		// if seconds reaches 60 
		if (seconds == 60) { 
		
			// increment minutes 
			minutes++; 

			// if minutes reaches 60 
			if (minutes == 60) { 
		
				// increment hours 
				hours++; 
				minutes = 0; 
			} 
			seconds = 0; 
		} 
	} 
} 

// Driver Code 
int main() 
{ 
	// start timer from 00:00:00 
	timer(); 
	return 0; 
}