PHP DATE PROBLEM

<?php
mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
echo "<link rel='stylesheet' href='..\styles.css'>";

require('library/php-excel-reader/excel_reader2.php');
require('library/SpreadsheetReader.php');
require('db_config.php');


if(isset($_POST['Submit'])){


  $mimes = ['application/vnd.ms-excel','application/vnd.openxmlformats-officedocument.spreadsheetml.sheet'];
  if(in_array($_FILES["file"]["type"],$mimes)){


    $uploadFilePath = 'uploads/'.basename($_FILES['file']['name']);
    move_uploaded_file($_FILES['file']['tmp_name'], $uploadFilePath);


    $Reader = new SpreadsheetReader($uploadFilePath);


    $totalSheet = count($Reader->sheets());


    echo "You have total ".$totalSheet." sheets<br><br>".


    $html="<table border='2px solid black'>";
    $html.="<tr><th>Description</th><th>Date</th><th>Amount</th><th>Merchant</th><th>Type</th><th>Source</th></tr>";
$query = "Insert Into main(Description,Date,Amount,Merchant,Type,  Source) Values(?, ?, ?, ?, ?, ?)";
$stmp = $mysqli->prepare($query);

    /* For Loop for all sheets */
    for($i=0;$i<$totalSheet;$i++){


      $Reader->ChangeSheet($i);
      foreach ($Reader as $Row)
      {
        $html.="<tr>";
        $des = isset($Row[0]) ? $Row[0] : '';
        $d = isset($Row[1]) ? $Row[1] : '';
     $timestamp = DateTime::createFromFormat('d/m/Y',$d);
  $d1 = $timestamp->format('Y-m-d');
    $a = isset($Row[2]) ? $Row[2] : '';
    $m = isset($Row[3]) ? $Row[3] : '';
    $t = isset($Row[4]) ? $Row[4] : '';
    $s = isset($Row[5]) ? $Row[5] : '';
if($des == '' || $d == '' || $a == '' || $m == '' || $s == '' || $t == ''){continue;}
else{
        $html.="<td>".$des."</td>";
        $html.="<td>".$d1."</td>";
    $html.="<td>".$a."</td>";
    $html.="<td>".$m."</td>";
    $html.="<td>".$t."</td>";
    $html.="<td>".$s."</td>";
        $html.="</tr>";

    $stmp->bind_param('sissss', $des, $d1, $a, $m, $t, $s);
    $stmp->execute();
       }


    }


    $html.="</table>";
    echo $html;
if(mysqli_error($mysqli) == ''){
    echo "<br />Data Inserted in dababase";}
else{echo "Error: ";
    echo mysqli_error($mysqli);
}};}
else {die("<br/>Sorry, Your File type is not allowed. You can only use a <code>.xlsx</code> file or a <code>.xls</code> file.");
  }


}


?>