Javascript String reformatPlaytime()

/// Prototype declarations
String.prototype.reformatPlaytime = function () {
    var sec_num = parseInt(this, 10); // don't forget the second param
    var hours   = Math.floor(sec_num / 3600);
    var minutes = Math.floor((sec_num - (hours * 3600)) / 60);
    var seconds = sec_num - (hours * 3600) - (minutes * 60);

    if (hours   < 10) {hours   = "0"+hours;}
    if (minutes < 10) {minutes = "0"+minutes;}
    if (seconds < 10) {seconds = "0"+seconds;}
    var time    = hours+':'+minutes+':'+seconds;
    return time;// w  w w .  j a va  2  s  .  c  o  m
}

function reformat_playtime(input){
    input = Math.round(input);
    return input.toString().reformatPlaytime();
}