Date Validation

This can be done by adding some extra code to a new plugin or your theme functions.php. Something like this:

add_filter('frm_validate_field_entry', 'my_custom_validation', 10, 3);
function my_custom_validation($errors, $posted_field, $posted_value){
  if($posted_field->id == 25){ //change 25 to the ID of the start date field
    if(strtotime($_POST['item_meta'][29]) < strtotime($posted_value)){ //if value of field 29 is less than field 25, then it's wrong. Change 29 to the ID of your end date field
       //if it doesn't match up, add an error:
       $errors['field'. $posted_field->id] = 'That field is wrong!';
    }
 }
  return $errors;
}