Grocery CRUD: To change the integer time to a timestamp when state=Read

Question

While using CRUD, I have a database field "created"(dataType=>integer).

When state='list' the following code converts the integer to date format and display:

$crud->callback_column('created', array($this,'date_display'));       //where date_display a function to change date format works


 function date_display($value, $row){
    return  date('d M Y H:i:s',$value);
 }

Whereas when state='read' the above code does not work and neither the function change_field_type works:

  $crud->change_field_type('created', 'datetime');

How to change the integer time to date format in case of state='read'?