$form = new Zend_Form();
$mockDate = new Zend_Form_Element_Text('mock');
$mockDate->addValidator(???????);
$form->addElements(array($mockDate));
$result = $form->isValid();
if ($result) echo "YES!!!";
else echo "NO!!!";
假设元素采用日期格式。如何确定给定的日期大于或等于今天?