function showTime() {
    'use strict';

    var now = new Date(),
    hours = now.getHours(),
    minutes = now.getMinutes(),
    seconds = now.getSeconds();

    if (hours < 10){
        hours = '0' + hours;
    }
    if (minutes < 10){
        minutes = '0' + minutes;
    }
    if (seconds < 10){
        seconds = '0' + seconds;
    }
    document.getElementById('clock').textContent = hours + ':' + minutes + ':' + seconds;
};
window.onload = function(){
    'use strict';
    setInterval(showTime, 500);
};