자바스크립트에서 숫자를 시간단위(시,분,초)로 변경하는 방법은?

String.prototype.toHHMMSS = function () {
    var myNum = parseInt(this, 10);
    var hours   = Math.floor(myNum / 3600);
    var minutes = Math.floor((myNum - (hours * 3600)) / 60);
    var seconds = myNum - (hours * 3600) - (minutes * 60);

    if (hours   < 10) {hours   = "0"+hours;}
    if (minutes < 10) {minutes = "0"+minutes;}
    if (seconds < 10) {seconds = "0"+seconds;}
    return hours+':'+minutes+':'+seconds;
}

time01 = '60';
time01.toHHMMSS();

"00:01:00"

time01 = '100';
time01.toHHMMSS();

"00:01:40"