function showTime() {
    var today = new Date();
    var BigDay = new Date("august 20, 2015, 16:25:00")
    var msPerDay = 24 * 60 * 60 * 1000;
    var timeLeft = (BigDay.getTime() - today.getTime());
    if (timeLeft < 0) {
        return;
    }
    var daysLeft = Math.floor(timeLeft / msPerDay);
    timeLeft -= daysLeft * msPerDay;
    var hrsLeft = Math.floor(timeLeft / msPerDay * 24);
    timeLeft -= hrsLeft * msPerDay / 24;
    var minsLeft = Math.floor(timeLeft / msPerDay * 24 * 60);
    timeLeft -= minsLeft * msPerDay / 24 / 60;
    var secLeft = Math.floor(timeLeft / 1000);
    
    var display = '';
    if (daysLeft > 0) {
        display += daysLeft + ' d ';
    }
    if (hrsLeft > 0) {
        display += hrsLeft + ' h ';
    }
    if (minsLeft > 0) {
        display += minsLeft + ' m ';
    }
    if (secLeft > 0) {
        display += secLeft + ' s ';
    }

    document.getElementById('vysledek').innerHTML = display;
    timerID = setTimeout(showTime, 1000);
}


function stopClock() {
    if (timerID) {
        clearTimeout(timerID);
        timerID = null;
    }
}
function startClock() {
    stopClock();
    showTime();
}

var timerID = null;