class User extends Model {
  public $timestamps = false;//关闭自动维护
  public static function boot() {
    parent::boot();
    #只添加created_at不添加updated_at
    static::creating(function ($model) {
      $model->created_at = $model->freshTimestamp();
      //$model->updated_at = $model->freshTimeStamp();
    });
  }
}

此处有坑:使用create方法创建一条记录时返回值的created的值是这样的： 
“created_at”: { 
“date”: “2017-09-27 13:47:12.000000”, 
“timezone_type”: 3, 
“timezone”: “Asia/Shanghai” 
}, 
并不是想象中的 
“created_at”: “2017-09-27 13:49:39”, 

class User extends Model {
  const UPDATED_AT = null;//设置update_at为null
  //const CREATED_AT = null;
}

此处有坑:使用destroy删除会报错 
Missing argument 2 for Illuminate\Database\Eloquent\Model::setAttribute() 
使用delete不影响,wherein也不影响

class User extends Model {
  //重写setUpdatedAt方法
  public function setUpdatedAt($value) {
    // Do nothing.
  }
  //public function setCreatedAt($value)
  //{
    // Do nothing.
  //}
}

class User extends Model {
  //重写setUpdatedAt方法
  public function setUpdatedAtAttribute($value) {
    // Do nothing.
  }
  //public function setCreatedAtAttribute($value)
  //{
    // Do nothing.
  //}
}

class CreatePostsTable extends Migration {
  public function up() {
   Schema::create('posts', function(Blueprint $table) {
   $table->timestamp('created_at')
   ->default(DB::raw('CURRENT_TIMESTAMP'));
  });
}