Phalcon Form Element with several values

I try to make a \Phalcon\Forms\Element with a date-range picker. How do I bind it to several fields in the model?

In my model I have:

$model->time_start (datetime)

$model->time_end (datetime)

I want to create a DateTimeInput element with the ID "time", that is going to load the default values from model with fields "time_start" and "time_end". The problem is also with $form->bind($_POST, $entity);

class DateTimeInput extends \Phalcon\Forms\Element\Text {

    public function __construct($elementName) {
        $this->setName($elementName);
    }

    public function render($attributes = null) {
        $attributes = $this->prepareAttributes($attributes);
        if(!empty($attributes)) {
            foreach($attributes as $attrName => $attrValue) {
                $this->setAttribute($attrName, $attrValue);
            }
        }

        $attributes = $this->getAttributes();
        $date_format = (!empty($attributes) && array_key_exists('data-date-format', $attributes) ? $attributes['data-date-format'] : 'dd.mm.yyyy');
        $html = '<div class="input-group input-large date-picker input-daterange" data-date-format="'.$date_format.'"><input type="text" name="'.$this->getName().'_start"'; // data-date="10/11/2012"

        // write every attributes of select element in DOM

        $input_id = false;
        if(!empty($attributes)) {
            foreach($attributes as $attrName => $attrValue) {
                if($attrName == '0') { $attrName = 'id'; $input_id = $attrValue.'_start'; }
                if($attrName == 'class') $attrValue .= ' input_postcode';
                if($attrName == 'data-date-format') continue;
                $html .= ' '.$attrName.'="'.$attrValue.'"';
            }
        }
        $html .= ' />';

        $html .= '<span class="input-group-addon"> '._('to').' </span><input type="text" name="'.$this->getName().'_end"'; // data-date="10/11/2012"

        // write every attributes of select element in DOM
        $attributes = $this->getAttributes();
        $input_id = false;
        if(!empty($attributes)) {
            foreach($attributes as $attrName => $attrValue) {
                if($attrName == '0') { $attrName = 'id'; $input_id = $attrValue.'_end'; }
                if($attrName == 'class') $attrValue .= ' input_postcode';
                $html .= ' '.$attrName.'="'.$attrValue.'"';
            }
        }
        $html .= ' />';

        $html .= '</div>';

        \Phalcon\Di::getDefault()->get('assets')->collection('css_extra')
            ->addCss('assets/global/plugins/bootstrap-datepicker/css/datepicker3.css')
            ->addCss('assets/global/plugins/bootstrap-daterangepicker/daterangepicker-bs3.css')
            ->join(false);
        \Phalcon\Di::getDefault()->get('assets')->collection('js_extra')
            ->addJs('assets/global/plugins/bootstrap-datepicker/js/bootstrap-datepicker.js')
            ->addJs('assets/global/plugins/bootstrap-daterangepicker/daterangepicker.js')
            ->join(false);

        //&callback=?
        $html .= '<script type="text/javascript">
            $(function() {
                if (jQuery().datepicker) {
                    $(\'.date-picker\').datepicker({
                        rtl: Metronic.isRTL(),
                        orientation: "left",
                        autoclose: true
                    });
                }
            });
        </script>';
        return $html;
    }

}

In my Form:

class ProjectForm extends \Phalcon\Forms\Form {

    public function initialize() {
        $timespan = new DateTimeInput("time");
        $timespan->setLabel(_('Timespan'));
        $timespan->setAttribute('data-date-format', 'dd.mm.yyyy');
        $this->add($timespan);
    }
}