Lithium and validating complex form inputs - how?

I've done quite a few Lithium tutorials (links below in case they help someone else, and also to show I've done my homework:) and I understand the most basic parts of creating models, views, controllers and using MVC to create a DB record based on form input.

However, I'm new to MVC for webapps and Lithium, and I'm not sure how I should write my code in more complicated situations. This is a general question, but two specific validation questions that I have are:

• How should I validate date data submitted from the form?
• How should I check that the two user email fields have the same value?

I would be very grateful for any help with these questions, and concrete examples like this will also really help me understand how to do good MVC coding in other situations as well!

Date entry - validating data split across multiple form inputs

For UI reasons, the sign up form asks users to enter their DOB in three fields:

<?=$this->form->field('birthday', array('type' => 'select', 'list' => array(/*...*/))); ?>
<?=$this->form->field('birthmonth', array('type' => 'select', 'list' => array(/*...*/))); ?>
<?=$this->form->field('birthyear', array('type' => 'select', 'list' => array(/*...*/))); ?>

What is the best way to validate this server-side? I think I should take advantage of the automagic validation, but I'm not sure of the best way do that for a set of variables that aren't really part of the Model. E.g.:

• Should I post-process the $this->request->data in UsersController? E.g. modify $this->request->data inside UsersController before passing it to Users::create.
• Should I pull the form fields out of $this->request->data and use a static call to Validator::isDate inside UsersController