Timer in C++ using system calls

// 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()
{
    // infinite 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;
}