Zend FR

class JS_Validate_CompareDates extends Zend_Validate_Abstract{
      const MSG_DATEBEFORE = 'msgDateBefore';
      public $date = null;
      
      protected $_messageTemplates = array(
              self::MSG_DATEBEFORE => "End date must be start date"
      );
       
       /**
        * The fields the current element needs to compare
        * 
        * @var array
        */
       protected $_fieldsToCompare = array();
       
       /**
        * constructor of this validator
        * The argument of this constructor is the third element of the elements addValidator
        * method. 
        * @return unknown_type
        */
       public function __construct($fieldsToCompare =array()){
               if(is_array($fieldsToCompare)){
                       foreach($fieldsToCompare as $field){
                               $this->_fieldsToCompare[] = (string) $field;
                       }
               }else{
                       $this->_fieldsToCompare[] = (string) $fieldsToCompare;
               }
               
               
       }
       /**
        * check if the element using this validator is valid.
        * 
        * This method will compare the $value of the element to the other elements 
        * it needs to match. If they all match, the method returns true.
        * 
        * @param $value string
        * @param $context array  All other elements from the form
        * @return boolean Returns true if the element is valid.
        * @see Validate/Zend_Validate_Interface#isValid($value)
        */
       public function isValid($value, $contexts = null){
               $value = (string)$value;
               $this->_setValue($value);
               
               $error = false;
               var_dump($value);
               var_dump($this->_fieldsToCompare);
               exit;
               foreach($this->_fieldsToCompare as $fieldname){
                       
                       if(!isset($context[$fieldname]) || $value !== $context[$fieldname]){    
                               $error = true;
                               $this->_error(self::MSG_DATEBEFORE);
                               break;
                       }
               }
               return !$error;
               
               $date1 = new Zend_Date();
               $date1->set($value, Zend_Date::DATES);
               
               var_dump($date1);
               $date2 = new Zend_Date();
               $date2->set($this->getMinDate(), Zend_Date::DATES);
               var_dump($date2);
               $date = $date1->compareDate($date2,Zend_Date::DATES);
               var_dump($date);
               exit;
       }
       
       public function setMinDate($value){
               $this->date = $value;
               
       }
       
       public function getMinDate(){
               return $this->date;     
       }
       
       

 }

  and my form:

  <?php
  class JS_Form_NewEventForm extends ZendX_JQuery_Form{

       public function init($options=null){
               //parent::__construct($options);
               
               $this->setName("newEvent");
               $this->addElementPrefixPath('JS_Validate','JS/Validate/','validate');           
               //naam
                $evtName = new Zend_Form_Element_Text("evt_name");
                $evtName->setLabel("Evenement Naam: ")
                                ->setRequired(true);
                // omschrijving
                $evtDescription = new Zend_Form_Element_Textarea("evt_description",array("rows"=>6,"cols"=>25));
                $evtDescription->setLabel("Evenement omschrijving: ");
                
                // locatie
                
                $evtAdr = new Zend_Form_Element_Select("adr_id");
                $evtAdr->setLabel("Locatie: ");
                $locaties = $this->getLocations();
                $evtAdr->setMultiOptions($locaties);
                
                $newAdr = new Zend_Form_Element_Button("new_adr");
                $newAdr->setLabel("+");
                // begin datum  
                $evtStartDate = new ZendX_JQuery_Form_Element_DatePicker("evt_startdate",array("label"=> "Begin Datum"));
                $evtStartDate->setJQueryParam('dateFormat', 'dd-mm-yy');
                $evtStartDate->addValidator(new Zend_Validate_Date('DD-MM-YYYY'));
                $evtStartDate->addValidator('CompareDates',false,array($evtEndDate));
                // date validator
                
                
                
                // eind datum
                $evtEndDate = new ZendX_JQuery_Form_Element_DatePicker("evt_enddate",array("label"=> "Eind Datum"));
                $evtEndDate->setJQueryParam('dateFormat', 'dd-mm-yy');
                $evtEndDate->addValidator(new Zend_Validate_Date('DD-MM-YYYY'));
                $evtEndDate->addValidator('CompareDates',false,array($evtStartDate));
                // begin tijd
                $evtStartTime = new Zend_Form_Element_Text("evt_starttime");
                $evtStartTime->setLabel("Begin Tijd");
                $evtStartTime->addValidator(new Zend_Validate_Date('hh:mm',new Zend_Locale('auto')));
                
                // eind tijd
                $evtEndTime = new Zend_Form_Element_Text("evt_endtime");
                $evtEndTime->setLabel("Eind tijd");
                $evtEndTime->addValidator(new Zend_Validate_Date('hh:mm'));
                // aantal personen
                $amountPersons = new Zend_Form_Element_Text("evt_amtpersons");
                $amountPersons->setLabel("Aantal personen nodig ");
                                        
                $save = new Zend_Form_Element_Submit("save");
                $save->setLabel("Opslaan");
                
                
                $this->addElements(array($evtName,$evtDescription,$evtAdr,$newAdr,$evtStartDate,$evtEndDate,$evtStartTime,$evtEndTime,$amountPersons,$save));
                $this->setMethod('post');
                $this->setAction(Zend_Controller_Front::getInstance()->getBaseUrl().'/events/add');
        
        
        }
        private function getLocations(){
                
                $adrTable = new JS_Model_DbTable_addresses();
                $rows = $adrTable->getAddressList();
                return $rows;
        }
        
        
  }