Based on an example in a previous blog post (seen here) Matthew Weier O'Phinney wanted to clear a few things up on the "date of birth" element he had mocked up in his Zend_Form example.

In my last post on decorators, I had an example that showed rendering a "date of birth" element [...]. This has prompted some questions about how this element might be represented as a Zend_Form_Element, as well as how a decorator might be written to encapsulate this logic. Fortunately, I'd already planned to tackle those very subjects for this post!

To be able to use the element in its current state the key lies in the setValue method. More correctly in the overriding of the setValue method. He includes an example class that is smart enough to use that custom form element. It has get and set methods for each of the date fields (month/day/year) and the set/getValue methods that can interact using them. He wraps this all up inside a form decorator and creates an instance of the Date element to help create and handle the properties it has.