function exportValue(&$submitValues, $assoc = false) {
        $valuearray = array();
        foreach ($this->_elements as $element){
            $thisexport = $element->exportValue($submitValues[$this->getName()], true);
            if ($thisexport!=null){
                $valuearray += $thisexport;
            }
        }
        if (count($valuearray) && isset($valuearray['year'])) {
            if($this->_options['optional']) {
                // If checkbox is on, the value is zero, so go no further
                if(empty($valuearray['enabled'])) {
                    return $this->_prepareValue(0, $assoc);
                }
            }
            // Get the calendar type used - see MDL-18375.
            $calendartype = \core_calendar\type_factory::get_calendar_instance();
            $gregoriandate = $calendartype->convert_to_gregorian($valuearray['year'], $valuearray['month'], $valuearray['day']);
            $value = make_timestamp($gregoriandate['year'],
                                                      $gregoriandate['month'],
                                                      $gregoriandate['day'],
                                                      0, 0, 0,
                                                      $this->_options['timezone'],
                                                      true);

            return $this->_prepareValue($value, $assoc);
        } else {
            return null;
        }
    }