关于Carbon的疑问

laravel版本是8.5，例子1和例子2中，都是加7天，为什么$user->created_at本身没有变化，是因为laravel自动把模型字段的Carbon类型转为CarbonImmutable类型了吗？

例子1

 use Carbon/Carbon;
 use App/Models/User;

 $user = new User;
 $user->created_at = Carbon::now();
 \Log::info($user->created_at->toDateTimeString());//2022-04-29 11:31:28
 $user->created_at->addDays(7);
 \Log::info($user->created_at->toDateTimeString());//还是 2022-04-29 11:31:28

例子2

 use Carbon/Carbon;

 $now = Carbon::now();
 \Log::info($now->toDateTimeString());//2022-04-29 11:31:28
 $now->addDays(7);
 \Log::info($now->toDateTimeString());//变成了 2022-05-06 11:31:28