overriding submit functions with content types
One way to do it is by using hook_form_alter, we can able to define a custom submit handler.

Find the form_id of that form and define a custom submit handler.

hook_form_alter(&amp;$form, &amp;$form_state, $form_id){
  if($form_id == 'custom_add_user_form'){
        $form['actions']['submit']['#submit'][] = 'show_date_submit_handler'; 
        //for submit $form['actions']['submit']  
        //for preview $form['actions']['preview']   
    }
}



function show_date_submit_handler(&amp;$form, &amp;$form_state){
    // manipulate the form submitted values over here
   }