Unable to save unix timestamp

[ahmed130544]

I have 2 integer columns "created_at" and "updated_at", in which want to save UNIX timestamp value, but unable to do.
Tried multiple ways like model boot method, tap method of activity logger, and createdAt method as well, but nothing worked.

Following are my logic to save activity log:

activity($this->data['log_type'])
->performedOn($this->data['model'])
->causedBy($this->data['user'])
->tap(function(Activity $activity) {
    $activity->created_at = time();
})
->createdAt(Carbon::now())
->withProperties(['topic_num' => $this->data['topic_num'], 'camp_num' => $this->data['camp_num'], 'url' => $this->data['url']])
->log($this->data['activity'] . ' by ' . $this->data['nick_name']);