App\Article {#690
     published_at: Carbon\Carbon {#689
       +"date": "2015-10-11 14:46:10",
       +"timezone_type": 3,
       +"timezone": "UTC",
     },
     updated_at: "2015-10-11 14:46:10",
     created_at: "2015-10-11 14:46:10",
     id: 1,
   }

再试试
App\Article {#690 published_at: Carbon\Carbon {#689

试试。

$article=App\Article::create(['title'=>'3','content'=>'2','published_at'=>Carbon\Carbon::now()]);