Timer in C++ using system calls

The task is to create timer without using any graphics or animation. The timer will be made using system calls wherever necessary. Timer in this context means a stopwatch with up-counting of time.
The timer is created in Linux. Following system calls of Linux are used: 
sleep() : It will make the program sleep for number of seconds provided as arguments to the function. 
system() : It is used to execute a system command by passing the command as argument to this function.

Below is the implementation for creating timer using System Calls: 

CPP

// 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;
}

Output:

Note: This can be made to run on Windows with a bit of modifications. 
Modifications required: 
1. Use “cls” in place of “clear” in system() call. 
2. Use ‘S’ in sleep() function in place of lower case ‘s’ in sleep() function. 
3. Include windows.h header file.
Making these changes code should run perfectly fine on Windows.