form validation time fields with formidablepro plugin

I have formidablepro over Wordpress. I want to create a validation in one field with a time that must be major of other field, i mean:

START_TIME > END_TIME (START_TIME has form id 23 and END_TIME form id 24)

I know that i need to add code in functions.php theme, 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 == 24){ 
    XXXXXXXXXXXXX{ 
    $errors['field'. $posted_field->id] = 'That field is wrong!';
    }
  }
  return $errors;
}

Any clue in the form validation?

via Chebli Mohamed