Here is a Javascript function that will tell you if 2 dates make a valid date range.

function isValidDateRange( objstartMonth,objstartDay, objstartYear, objendMonth,objendDay, objendYear) 
{ 

var startDate = new Date(objstartYear.options[objstartYear.selectedIndex].value, objstartMonth.options[objstartMonth.selectedIndex].value, objstartDay.options[objstartDay.selectedIndex].value); 
var endDate = new Date(objendYear.options[objendYear.selectedIndex].value, objendMonth.options[objendMonth.selectedIndex].value, objendDay.options[objendDay.selectedIndex].value); 

if (startDate >= endDate){ 
     alert("Invaild Date Range");
     return false; 
}
else{
     return true;
}

}