This is how easy Zend_Form->persistDate() is

zend_wtform.php

    public function persistData($namespace = 'My_Super_Awesome_Form')
    {
        if (null === $this->_session) {
            $this->_session = new Zend_Session_Namespace($namespace);
        }
        
        if ($this->_session->values) {
            $this->populate($this->_session->values);
        }
        
        $this->_session->values = $this->getValues();
    }