get you Date() into hhmmss.msec

hhmmss.js

var zeroPad2 = function (n) {
    var _n = n + "";
    if (_n.length < 2) {
        return "0" + _n;
    } else {
        return _n;
    }
};
var zeroPad = function (n, width) {
    var z = "0";
    var _n = n + "";
    return _n.length >= width ? _n : new Array(width - _n.length + 1).join(z) + _n;
};



var timestampToHHmmSS = function(dt){
  var hours = Math.trunc(dt.getUTCHours());
  var mins = Math.trunc(dt.getUTCMinutes());
  var sec = Math.trunc(dt.getUTCSeconds());

  var msec = zeroPad(Math.trunc(dt.getUTCMilliseconds()), 3);
  
  return zeroPad2(hours) + ":" + zeroPad2(mins) + ":" + zeroPad2(sec) + "." + msec;
}

var sec = 42;
var dt = new Date(sec * 1000)
timestampToHHmmSS(dt)