Installer: Fix invalid join time of admin user

Fixes #1664.

src/Install/Console/InstallCommand.php

@@ -11,6 +11,7 @@
 
 namespace Flarum\Install\Console;
 
+use Carbon\Carbon;
 use Exception;
 use Flarum\Console\AbstractCommand;
 use Flarum\Database\DatabaseMigrationRepository;
@@ -294,7 +295,7 @@ class InstallCommand extends AbstractCommand
             'username' => $admin['username'],
             'email' => $admin['email'],
             'password' => (new BcryptHasher)->make($admin['password']),
-            'joined_at' => time(),
+            'joined_at' => Carbon::now(),
             'is_email_confirmed' => 1,
         ]);