The prototype-solution from Krishna Chytanya is very nice, but needs a minor but important improvement.
The days param must be parsed as Integer to avoid weird calculations when days is a String like "1". (I needed several hours to find out, what went wrong in my application.)
Date.prototype.addDays = function(days) {
this.setDate(this.getDate() + parseInt(days));
return this;
};
Even if you do not use this prototype function:
Always be sure to have an Integer when using setDate().
