You can try to follow these steps for the customization, edit "includes/post-functions.php" file and go to line 720 for these code :
$validation_result = validate_element($target_input,$rules);
if($validation_result !== true){
$error_elements[$element_id] = $validation_result;
}
then put these code bellow that line
if (!empty($target_input['element_date'])) {
if($form_id == 16 && $element_id == 1){
$selected_date = date('Ymd',strtotime($target_input['element_date']));
$today_date = date('Ymd');
if($selected_date < $today_date){
$error_elements[$element_id] = "Can not select previous date";
}
}
}
in there I assume your form id is 16 with date element id 1. Don't forget to change those id's with yours.
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