Javascript Date getDaysInMonthOfYear()

Date.prototype.isLeapYear = function () {return this.getFullYear()%4?0:this.getFullYear()%100?1:this.getFullYear%400?0:1}
Date.prototype.getDaysInMonthOfYear = function(){
 month = this.getMonth();/*  w  w w.  j a v  a  2s . c  o m*/
 if(month === 1){
  return this.isLeapYear()? 29:28;
 }else{
  if(month>=8){
   return month%2? 31:30;
  }else{
   return month%2? 31:30;
  }
 }
}
Date.prototype.addDays = function(days){
 return new Date(this.getTime() + days*24*60*60*1000);
}
Date.prototype.addMonths = function(months){
 return new Date(this.getTime() + months*this.getDaysInMonthOfYear()*24*60*60*1000);
}

Date.prototype.addYears = function(year){
 return new Date(this.getTime() + year*24*60*60*1000*(this.isLeapYear()?366:365));
}