var TargetTime = document.getElementById("lblRemainingTime").innerHTML;
var sec = ConvertToSec(TargetTime);

function ConvertToSec(strTime) {
    var arrTime = strTime.split(':');
    return (arrTime[0] * 3600) + (arrTime[1] * 60) + (arrTime[2]) * 1;
}

function ConvertToTimeString(sec) {
    var Hour = sec / 3600;
    var Min = (sec % 3600) / 60;
    var Second = (sec % 3600) % 60;
    var time = pad(parseInt(Hour), 2) + ':' + pad(parseInt(Min), 2) + ':' + pad(parseInt(Second), 2);
    return time;
}

function CountDown() {
    var time = ConvertToTimeString(sec);
    document.getElementById("lblRemainingTime").innerHTML = time;
    if (sec > 0) {
        setTimeout("CountDown()", 1000);
        sec--;
    }
}

function pad(number, length) {

    var str = '' + number;
    while (str.length < length) {
        str = '0' + str;
    }

    return str;

}

CountDown();