JavaScript验证的跨浏览器问题

所以我让我的代码在Chrome中工作得很好。在FF中，我得到了带有日期的前两个警报，它们的格式正确，但如果选择了过去的日期，则不会抛出第三个警报。在IE中，我得到所有三个警报，但即使我选择了一个未来的日期。正如我所提到的，Chrome可以正常工作，所以如果有人能提供一些见解，使它在所有这些工作，那将是伟大的。

 function check()
{
   var selecteddate = document.form1.selectmonth.value + "-" + document.form1.selectday.value + "-" + document.form1.selectyear.value;
   var d = new Date();
   var today = (d.getMonth()+1) + "-" + d.getDate() + "-" + d.getFullYear();
   // I'd then make some alerts and it'd return the selected date and today with no problem
   alert(selecteddate);
   alert(today)
   //Now for the if statement is where it just stops working. I think maybe I'm doing
   //something wrong just solely in the if statement.
     if(new Date(selecteddate) < new Date(today))
       {
       alert("Past dates are not valid");
       return false;
       }
}
</script>