function add_weeks(dt, n)   
 {  
 return new Date(dt.setDate(dt.getDate() + (n * 7)));        
 }  
  
dt = new Date();  
console.log(add_weeks(dt, 10).toString());    
  
dt = new Date(2014,10,2);  
console.log(add_weeks(dt, 10).toString());