On the WebInOne blog there's a post about doing some date validation in CodeIgniter 2 forms using the "set_validation" method.

I wrote a tutorial here about the CI date. At that post I used the HTML <select> for the input of date. Sometime our client can want to use text box for the date input. We need the validation for this date. I have found this code from here. It is for CI 1.7.* and PHP 5.2.*. So I change some code for the CI 2.0.* and PHP 5.3.*.

There's some code snippets included in the post showing how to use it in the form (HTML), setting the validation on the form object (for US and UK validation) and the actual code of the updated custom Validation class.