In the example below note that date2 object was not deliberately changed after it was initially set, but still at the end of the script it gets a new value which is the same as the changed date1 object.
var date1=new Date();
alert(date2); //shows Sun Aug 17 11:46:50 UTC+0200 2008
alert(date2); //shows Mon Jun 9 11:46:50 UTC+0200 2008
It also works in the opposite direction: if you change date2, the date1 will also be changed.