PHP script send scheduled firebase notification

I need to schedule the push notifications that offered by firebase cloud messaging by using a PHP code. I'm currently using this code to push notifications instantly, but sometimes I need to push notifications at specific time.

function push_notification_android($device_id,$message){

    //API URL of FCM
    $url = 'https://fcm.googleapis.com/fcm/send';

    /*api_key available in:
    Firebase Console -> Project Settings -> CLOUD MESSAGING -> Server key*/    $api_key = 'AAAAKZLje1I:APbGQDw8FD...TjmtuINVB-g';
                
    $fields = array (
        'registration_ids' => array (
                $device_id
        ),
        'data' => array (
                "message" => $message
        )
    );

    //header includes Content type and api key
    $headers = array(
        'Content-Type:application/json',
        'Authorization:key='.$api_key
    );
                
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
    $result = curl_exec($ch);
    if ($result === FALSE) {
        die('FCM Send Error: ' . curl_error($ch));
    }
    curl_close($ch);
    return $result;
}

where can I put the time for pushing the notification?