[jQuery] inArray not working with dates

Can someone tell me what I may be doing wrong here?

        var dateList = new Array();
        dateList.push(new Date(2009,7-1,29));
        dateList.push(new Date(2009,7-1,30));
        dateList.push(new Date(2009,7-1,31));
        var testdate = new Date(2009,7-1,30);
        alert($.inArray(testdate,dateList)); //returns -1  ????

tia,

Dave