[SOLVED] Update auto Create_at & Update_at columns at laravel 5.4

here some simple code i have. actuly i was trying to insert some Tag to my database and no problem to that.but once insert my tag "created_at" & "updated_at" columns not update and just NULL how can i update auto this fields? i don't have any model just migration here my tag migration:

public function up()
{
    Schema::create('tags', function (Blueprint $table) {
        $table->increments('id');
        $table->string('tag');
        $table->timestamps(); 
    });

via siros