Handling delayed notifications in Laravel, pt. 2

AppointmentReminder.php

<?php 

namespace App\Notifications;

use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Notifications\Notification;
use Illuminate\Queue\SerializesModels;

class AppointmentReminder extends Notification implements ShouldQueue
{
    use Queueable, SerializesModels;
  
    public $appointment;
  
    public function __construct(Appointment $appointment)
    {
        $this->appointment = $appointment;
        
        $this->delay($appointment->start_date_time)->subDay(1);
    }
  
    public function via($notifiable)
    {
        if($this->dontSend($notifiable)) {
            return [];
        }    

        return ['mail'];
    }

    public function dontSend($notifiable)
    {
        return $this->appointment->status === 'cancelled';
    }
  
    public function toMail()
    {
        // Return mail contents
    }
}

subDay is not undefined, can u fix this??