找出3天前的日期

var arrd_ymd = '11/15/2009';

arrd_y = new Date(arrd_ymd);

arrd_y = arrd_y.valueOf();

arrd_y = arrd_y - 3 * 24 * 60 * 60 * 1000; //今天3天前的日期

arrd_30 = new Date(arrd_y);

var bef = arrd_30.getFullYear()+'-'+(arrd_30.getMonth()+1)+ '-'+arrd_30.getDate();

==>bef = '2009/11/11'