Form validation: Callback not changing the input value

[eluser]Nis Sarup[/eluser]
As I read the user guide any callback in a form validation rule should change the input value if the result returned from the callback function is anything but TRUE or FALSE.

I can't get it to do that.

I have a rule:

Code:

$rules['birthday'] = 'trim|required|callback__validate_birthday';

The callback is being called. It returns a timestamp if the birthday is valid, FALSE if not.

But

Code:

$this->input->post('birthday')

stays the same value whatever the callback returns. Shouldn't it be set to the result returned from the callback? Or did I misunderstand something?