AR.php

<?php
use yii\behaviors\TimestampBehavior;

public function behaviors()
{
     return [
          'timestamp' => function() {
              $b = new TimeStampBehavior();
              $b->attributes = [
                  ActiveRecord::EVENT_BEFORE_INSERT => ['created_at', 'updated_at'],
                  ActiveRecord::EVENT_BEFORE_UPDATE => ['updated_at'],
              ];
              // ...
              return $b;      
          },
    ];
}