Javascript Date serialize_date()

Date.prototype.serialize_date = function(){
    var month =  (Number(this.getMonth()) + 1);
    if (month.toString().length === 1){
        month = "0" + month.toString();
    }/*from w  w  w .  j ava2 s  .  c om*/
    var day =  (Number(this.getDate()) + 1);
    if (day.toString().length === 1){
        day = "0" + day.toString();
    }
    return this.getFullYear() + "-" + month + "-" + day;
};