Date validation in javacript

validate date format e.g. 12-2-2004,12-11-08
function validateDate()
{
obj=//name of your object
msg=//your mesage comes here
var validNum = /^([0-9]{1,2})-([0-9]{1,2})-([0-9]{2,4})$/;
if (validNum.test(obj.value) == false)
{
alert(msg);
obj.focus();
//obj.select();
return false;
}
return true;
}