Attribution de variable de date Javascript

var date1 = new Date();  
date1.setFullYear(2011, 6, 1);  

//2011-07-01, ok  
console.log(date1);

//set date2 the same date as date1  
var date2 = date1;

//...

//now I'm gonna set a new date for date2  
date2.setFullYear(2011, 9, 8);

//2011-10-08, ok  
console.log(date2);

//2011-10-08, wrong, expecting 2011-07-01  
//I didn't assign a new date to date1  
//WHY is date1 changed?  
console.log(date1);

source d'informationauteur railOne