lastlogin shield

I'm trying to get the latest shield login.
I do the following:

PHP Code:

$tmpUser = auth()->getProvider();

$user = $tmpUser->findById($users[$i]->id);

$lastLogin = $user->lastLogin();
$tmp = $lastLogin->toArray();
echo'<pre>';print_r($tmp['date']->format('Y-m-d H:i'));echo'</pre>';
d($tmp['date']->format('Y-m-d H:i'));exit(); 


This returns me:


But when I want to save it to an array:

PHP Code:

$tmpUser = auth()->getProvider();

$user = $tmpUser->findById($users[$i]->id);

$lastLogin = $user->lastLogin();
$tmp = $lastLogin->toArray();
            
$userList[] = [
    'id' => $users[$i]->id,
    'user' => $users[$i]->username,
    'grupos' => $grupo,
    'lastLogin' => $tmp['date']->format('Y-m-d H:i'), 


it returns me this error:


and I don't understand why with d or print_r/var_dump it prints the value, but I can't assign it to the array