<script type="text/javascript">
 
// grab and split date values
var start = document.getElementById('start').value;
var sarr = start.split('/');
var sday = parseInt( sarr[0] );
var smo = parseInt( sarr[1] );
var syr = parseInt( sarr[2] );
 
var end = document.getElementById('end').value;
var earr = end.split('/');
var eday = parseInt( earr[0] );
var emo = parseInt( earr[1] );
var eyr = parseInt( earr[2] );
 
// here we must compare date values
validate();
 
function validate () {
  switch ( true ) {
    case syr > eyr:
    alert( 'please pick an end year later than start year' );
    return false;
    case smo > emo:
    alert( 'please pick an end month later than start month' );
    return false;
    case sday > eday:
    alert( 'please pick an end day later than start day' );
    return false;
    default:
    document.getElementById('my_form_id').submit();
  }
}
</script>