nambah hari pakai javascript

aku juga sempet tersendat di sini juga neh. gimana caranya agar menambah hari pake javascript. dan sama juga, setelah di cari-cari caranya. ada juga script untuk nambah hari :D. scriptnya gini nech

function myDate(year, month, day) {
 this.year = year;
 this.month = month;
 this.day = day;
}

function dateAdd(date, n) {
 date.day += n;
 while(date.day <=0 || date.day > daysOfMonth(date.year, date.month)) {
  if(date.day <= 0){
   prevMonth(date);
   date.day += daysOfMonth(date.year, date.month);
  } else {
   date.day -= daysOfMonth(date.year, date.month);
   nextMonth(date);
  }
 }
}

function daysOfMonth(year, month) {
 var days = new makeArray(12);

 days[1] = 31;  days[2] = 28;  days[3] = 31;
 days[4] = 30;  days[5] = 31;  days[6] = 30;
 days[7] = 31;  days[8] = 31;  days[9] = 30; 
 days[10] = 31; days[11] = 30; days[12] = 31;

 if(year % 4 == 0 && year % 100 != 0 || year % 400 == 0)
  days[2] = 29;

 return days[month];
}

function prevMonth(date) {
 if(date.month == 1) {
  date.year--; date.month = 12;
 } else {
  date.month--;
 }
}

function nextMonth(date) {
 if(date.month == 12) {
  date.year++; date.month = 1;
 } else {
  date.month++;
 }
}

function makeArray(n) {
 this.length = n;
 return this;
}