Edit in JSFiddle 

var timerID = null;
var timerRunning = false;

function stopclock () {
    if(timerRunning) clearTimeout(timerID);
    timerRunning = false;
}

function startclock () {
    stopclock();
    showtime();
}

function showtime() {

    now = new Date();
    var CurHour = now.getHours();
    var CurMinute = now.getMinutes();
    var CurMonth = now.getMonth();
    var CurDate = now.getDate();
    var CurSecond = now.getSeconds();
    var Hourleft = 23 - CurHour
    var Minuteleft = 59 - CurMinute
    var Secondleft = 59 - CurSecond
    var Monthleft = 5 - CurMonth
    var Dateleft = 20 - CurDate

    var obj = document.getElementById('left_time');
    event_day = new Date(2014,12,24,23,59,59)

    if((event_day.getTime() - now.getTime()) > 0){
        var leftStr = "<div>남은시간 : ";
        if(Dateleft > 0) leftStr += "<b>"+Dateleft+"</b>일 ";
        if(Hourleft > 0) leftStr += "<b>"+Hourleft+"</b>시간 ";
        if(Minuteleft > 0) leftStr += "<b>"+Minuteleft+"</b>분 ";
        leftStr += "<b>"+Secondleft+"</b>초</div>";
        obj.innerHTML = leftStr;
    }else{
        obj.innerHTML = '<div>이벤트 기간이 종료되었습니다</div>';
    }

    now = null;
    timerID = setTimeout("showtime()",1000);
    timerRunning = true;

}

<body bgcolor='#FFFFFF' onLoad="startclock();">

<div id='left_time'></div>

</body>

</html>

body {font-size: 9pt; font-family: 돋움,Arial; color: #58595B;}