Js calculate specified date

function   DateAdd(interval,number,date)  
{  
/*
  * Function: Implement the DateAdd function of VBScript.
  * Parameters: interval, string expression, indicating the time interval to be added.
  * Parameters: number, numeric expression, indicating the number of time intervals to be added.
  * Parameters: date, time object.
  * Returns: the new time object.
  *   var   now   =   new   Date();
  *   var   newDate   =   DateAdd( "d ",5,now);
  *---------------   DateAdd(interval,number,date)   -----------------
  */  
        switch(interval)  
        {  
                case   "y "   :   {  
                        date.setFullYear(date.getFullYear()+number);  
                        return   date;  
                        break;  
                }  
                case   "q "   :   {  
                        date.setMonth(date.getMonth()+number*3);  
                        return   date;  
                        break;  
                }  
                case   "m "   :   {  
                        date.setMonth(date.getMonth()+number);  
                        return   date;  
                        break;  
                }  
                case   "w "   :   {  
                        date.setDate(date.getDate()+number*7);  
                        return   date;  
                        break;  
                }  
                case   "d "   :   {  
                        date.setDate(date.getDate()+number);  
                        return   date;  
                        break;  
                }  
                case   "h "   :   {  
                        date.setHours(date.getHours()+number);  
                        return   date;  
                        break;  
                }  
                case   "m "   :   {  
                        date.setMinutes(date.getMinutes()+number);  
                        return   date;  
                        break;  
                }  
                case   "s "   :   {  
                        date.setSeconds(date.getSeconds()+number);  
                        return   date;  
                        break;  
                }  
                default   :   {  
                        date.setDate(d.getDate()+number);  
                        return   date;  
                        break;  
                }  
        }  
}  
   
var   now   =   new   Date();  
//Add five days.  
var   newDate   =   DateAdd( "d ",5,now);  
alert(newDate.toLocaleDateString())  
//Add two months.  
newDate   =   DateAdd( "m ",2,now);  
alert(newDate.toLocaleDateString())  
//add one year  
newDate   =   DateAdd( "y ",1,now);  
alert(newDate.toLocaleDateString())