Usei o comando
$conta->getDataAbertura()->get("month")
como mostrado na solução e recebi
Fatal error: Uncaught Error: Call to a member function get() on string in C:\inetpub\wwwroot\design-patterns\indexFiltro.php:36 Stack trace: #0 {main} thrown in C:\inetpub\wwwroot\design-patterns\indexFiltro.php on line 36?