Date.prototype.addDays

I did not find that method how can I add days to my date, so here is solution:

// add days
Date.prototype.addDays = function(days) {
var secsDay = 86400000; // there are 86400000 secs in 1 day
var time = this.getTime(); // number of milliseconds since midnight of January 1, 1970.
var newdate = new Date(time + (days * secsDay));
this.setDate(newdate.getDate());
}

If anybody has better solution, you are welcome!