public function instance_form_definition(\MoodleQuickForm $mform) {
        $field = $this->get_field();
        // Get the current calendar in use - see MDL-18375.
        $calendartype = \core_calendar\type_factory::get_calendar_instance();

        $config = $field->get('configdata');

        // Always set the form element to "optional", even when it's required. Otherwise it defaults to the
        // current date and is easy to miss.
        $attributes = ['optional' => true];

        if (!empty($config['mindate'])) {
            $attributes['startyear'] = $calendartype->timestamp_to_date_array($config['mindate'])['year'];
        }

        if (!empty($config['maxdate'])) {
            $attributes['stopyear'] = $calendartype->timestamp_to_date_array($config['maxdate'])['year'];
        }

        if (empty($config['includetime'])) {
            $element = 'date_selector';
        } else {
            $element = 'date_time_selector';
        }
        $elementname = $this->get_form_element_name();
        $mform->addElement($element, $elementname, $this->get_field()->get_formatted_name(), $attributes);
        $mform->setType($elementname, PARAM_INT);
        $mform->setDefault($elementname, time());
        if ($field->get_configdata_property('required')) {
            $mform->addRule($elementname, null, 'required', null, 'client');
        }
    }