Mutators with two date fields

Hi, I have in DB 3 fields, notification_option (string), expiration_date and notification_date.

The idea it´s, a user select a notification option from a select and I save the dates with the option.

Example:

public function setNotificationOptionAttribute($notification){
    $this->attributes['notification_option'] = $notification;
    $expiration = Carbon::parse($this->attributes['expiration_date']);

    dump("Before - Expiration Date: " . $this->attributes['expiration_date']);
 
    switch ($notification) {
        case '15_days_before':
            dump("15 days before");
            $this->attributes['notification_date'] = $expiration->subDay(15);
            dump("Expiration date: " . $expiration);
            dump("Notification date: " . $this->attributes['notification_date']);
            break;
     
        default:
            $this->attributes['notification_date'] = null;
            break;
    }
    // dd();
}

The problem it´s the expiration_date change too, instead only notification_date. The output it`s:

"Before - Expiration Date: 2017-01-17 18:56:10"

"15 days before"

"Expiration date: 2017-01-02 18:56:10"

"Notification date: 2017-01-02 18:56:10"

Thanks!