framework/behaviors/TimestampBehavior.php
public function init()
    {
        parent::init();

        if (empty($this->attributes)) {
            $this->attributes = [
                BaseActiveRecord::EVENT_BEFORE_INSERT => [$this->createdAtAttribute, $this->updatedAtAttribute],
                BaseActiveRecord::EVENT_BEFORE_UPDATE => $this->updatedAtAttribute,
            ];
        }
    }
public function init()
    {
        parent::init();

        if (empty($this->attributes)) {
            $this->attributes = [
                BaseActiveRecord::EVENT_BEFORE_INSERT => [$this->createdAtAttribute, $this->updatedAtAttribute],
                BaseActiveRecord::EVENT_BEFORE_UPDATE => $this->updatedAtAttribute,
            ];
        }
    }
{@inheritdoc}