const timer = document.querySelector('#timer');
const url = 'http://worldtimeapi.org/api/timezone/africa/Algiers';

fetch(url)
  .then(res => res.json())
  .then(data => {
    let datetime = new Date(data.datetime);

    setInterval(() => {
      let time = datetime.getTime();
      datetime = new Date(time + 1000);

      timer.textContent = datetime.toString().slice(16, 24);
    }, 1000);
  });