Added ISO 8601 date format (#638)

Author: Raza403

classes/element_helper.php

@@ -702,6 +702,7 @@ public static function get_date_formats(): array {
         $dateformats = [
             1 => userdate($date, '%B %d, %Y'),
             2 => userdate($date, '%B %d' . $suffix . ', %Y'),
+            5 => userdate($date, '%Y-%m-%d'),
         ];
 
         $strdateformats = [
@@ -759,6 +760,10 @@ public static function get_date_format_string(int $date, string $dateformat): st
                 case 4:
                     $certificatedate = userdate($date, '%B %Y');
                     break;
+                case 5:
+                    // ISO 8601 date format (YYYY-MM-DD).
+                    $certificatedate = userdate($date, '%Y-%m-%d');
+                    break;
                 default:
                     $certificatedate = userdate($date, get_string('strftimedate', 'langconfig'));
             }