遇到DateTime-> sub的麻烦

Question

I have quite a situation here, I'm currently trying to code something which will give reminders of certain events, or tasks. For that, I have a ajax function, which will check every 10 minutes if there is a reminder to display. It's working fine until I try to compare the reminder's date and today's date.

I'm accessing my database to fetch all the events which have the flag for the reminder, and I get the duration needed for this reminder (ex : 15 minutes, 3 hours, 2 weeks) through an already written function which will get me the duration in minutes.

The main problem is, when I try to substract the duration to my current date (to have the reminder's date), nothing is happening, the $date_rappel, is the same a the $date_current. I searched everywhere to see if I'm doing it wrong, but I can't see the problem.

I tried to trigger_error() and var_dump() every single variable I have in my code to see if something was odd, but everything seems normal.

$events = '[';

$zz=0;
for ($r=0;$r < count($array_search_result);$r++)
  {

  $data=new $class_name;
  $data->load($array_search_result[$r]);
  $rappel = get_simple_obj('reminder_table',$data->cle_antlist_rappels);

  $duree = $rappel['duree'];

  $tmp_date = strval($data->date_echeance);
  $tmp_heure = strval($data->heure_echeance);
  $date_and_time = $tmp_date." ".$tmp_heure;

  $date_current = new DateTime($date_and_time);

  $date_rappel = $date_current;

  if($duree < 60){
    $tmpinterval = 'PT'.$duree.'M';
    $interval = new DateInterval($tmpinterval); 
    $date_rappel->sub($interval);       
  }elseif($duree >= 60 && $duree < 1440){
    $duree = $duree / 60;
    $tmpinterval = 'PT'.$duree.'H';
    $interval = new DateInterval($tmpinterval); 
    $date_rappel->sub($interval);    
  }elseif($duree >= 1440 && $duree < 10080){
    $duree = $duree / 60 / 24;
    $tmpinterval = 'P'.$duree.'D';
    $interval = new DateInterval($tmpinterval);   
    $date_rappel->sub($interval);                        
  }else{
    $duree = $duree / 60 / 24 / 7;
    $tmpinterval = 'P'.$duree.'W';
    $interval = new DateInterval($tmpinterval);     
    $date_rappel->sub($interval);                    
  }

if($date_current < $date_rappel == true){
   // Add the event with the reminder to the ajax request                                                
}

$events .= $event;

$events .= ']';      

$returnSate = 'OK';
$status_response=0;

This is what the trigger_error of $date_current and $date_rappel are returning :

PHP Notice:  11:39:00 2019-05-14 in .../operation-reminder.php on line 308
PHP Notice:  11:39:00 2019-05-14 in .../operation-reminder.php on line 309