public function date(array $values)
    {
        if (count($values) != 1) {
            $vars = array('@field' => $this->translation->text('Condition'));
            return $this->translation->text('@field has invalid value', $vars);
        }

        $timestamp = strtotime(reset($values));

        if (empty($timestamp)) {
            $vars = array('@field' => $this->translation->text('Condition'));
            return $this->translation->text('@field has invalid value', $vars);
        }

        return true;
    }