code: public function getFunctions()
      {
          return array(
              new ExpressionFunction(
                  'date',
                  function ($value) {
                      return sprintf('date(%1$s, time())', $value);
                  },
                  function ($arguments, $str) {
                      return date($str, time());
                  }
              ),
          );
      }
docs: {@inheritdoc}