Já kdysi používal tohle, ani už nevím kde jsem to vzal

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var timerID = null;
var timerRunning = false;
function showtime() {
        today = new Date();
        BigDay = new Date("18:00, may 17, 2005")
        msPerDay = 24 * 60 * 60 * 1000 ;
        timeLeft = (BigDay.getTime() - today.getTime());
        e_daysLeft = timeLeft / msPerDay;
        daysLeft = Math.floor(e_daysLeft);
        e_daysLeft = timeLeft / msPerDay;
        daysLeft = Math.floor(e_daysLeft);
        e_hrsLeft = (e_daysLeft - daysLeft)*24;
        hrsLeft = Math.floor(e_hrsLeft);
        minsLeft = Math.floor((e_hrsLeft - hrsLeft)*60);
        hrsLeft = hrsLeft;
        e_minsLeft = (e_hrsLeft - hrsLeft)*60;
        secLeft = Math.floor(e_hrsLeft);
        secLeft = Math.floor((e_minsLeft - minsLeft)*60);
        secLeft = secLeft;
        Temp3='Do začátku kochlechatu zbývá: '+daysLeft+' dní,  '+hrsLeft+' hod., '+minsLeft+' min., '+secLeft+' sec.'
        document.frm.left.value=Temp3;
        timerID = setTimeout("showtime()",1000);
        timerRunning = true;
}
var timerID = null;
var timerRunning = false;
function stopclock () {
        if(timerRunning)
        clearTimeout(timerID);
        timerRunning = false;
}
function stopClock() {
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        return;
}
function startclock () {
        stopclock();
        showtime();
}
// -->
</script>