Add this snippet and check according to your needed.
add_action('acf/validate_save_post', 'my_acf_validate_save_post');
function my_acf_validate_save_post()
{
$start = $_POST['acf']['field_5fb0e816ea4fc'];
//$start = new DateTime($start);
$start = strtotime($start);
$end = $_POST['acf']['field_5fb0e83aea4fd'];
//$end = new DateTime($end);
$end = strtotime($end);
if( current_user_can('manage_options') ) {
acf_reset_validation_errors();
}
// check custom $_POST data
if ($start > $end ) {
acf_add_validation_error($_POST['acf']['field-600e609de8ab8'], 'End Date should be later than the Start Date');
}else if ($start == $end ) {
acf_add_validation_error($_POST['acf']['field-600e609de8ab8'], 'End Date should be equal to the Start Date');
}
}
Screenshot in Error :
