var decimalTimeString = "3,91201611110591";
var decimalTime = parseFloat(decimalTimeString);
decimalTime = decimalTime * 60 * 60;
var hours = Math.floor((decimalTime / (60 * 60)));
decimalTime = decimalTime - (hours * 60 * 60);
var minutes = Math.floor((decimalTime / 60));
decimalTime = decimalTime - (minutes * 60);
var seconds = Math.round(decimalTime);
if(hours < 10)
{
    hours = "0" + hours;
}
if(minutes < 10)
{
    minutes = "0" + minutes;
}
if(seconds < 10)
{
    seconds = "0" + seconds;
}
alert("" + hours + ":" + minutes + ":" + seconds);

var decimalTimeString = "3,91201611110591";
var n = new Date(0,0);
n.setSeconds(+decimalTimeString * 60 * 60);
alert(n.toTimeString().slice(0, 8));