<?php 
class App_Form_Validators_MyData extends Zend_Validate_Abstract
    {
        const INVALID_DATA_FORMAT = 'invalid_data_format';

	    protected $_messageTemplates = array(
	        self::INVALID_DATA_FORMAT=>'Formato data non valido'
	    );
     
        public function isValid($value)
        {
            $this->_setValue($value);
     
            $isValid = true;

            $dateValidatorTrattino = new Zend_Validate_Date(array('format' => 'yyyy-mm-dd'));
            if(!$dateValidatorTrattino->isValid($value)){
                $this->_error(self::INVALID_DATA_FORMAT);
                $isValid = false;
            }

            return $isValid;
        }
    }