diff --git a/core/lib/Drupal/Core/Datetime/DateHelper.php b/core/lib/Drupal/Core/Datetime/DateHelper.php
index ad49016c058a5e74163e46211d5bfc5a38d5431f..4cfc74640fae9892c5d4d10ca37eadecd135796f 100644
--- a/core/lib/Drupal/Core/Datetime/DateHelper.php
+++ b/core/lib/Drupal/Core/Datetime/DateHelper.php
@@ -254,14 +254,24 @@ public static function weekDaysAbbr1($required = FALSE) {
    *   An array of weekdays.
    *
    * @return array
-   *   An array of weekdays reordered to match the first day of the week.
+   *   An array of weekdays reordered to match the first day of the week. The
+   *   keys will remain unchanged. For example, if the first day of the week is
+   *   set to be Monday, the array keys will be [1, 2, 3, 4, 5, 6, 0].
    */
   public static function weekDaysOrdered($weekdays) {
     $first_day = \Drupal::config('system.date')->get('first_day');
     if ($first_day > 0) {
       for ($i = 1; $i <= $first_day; $i++) {
-        $last = array_shift($weekdays);
-        array_push($weekdays, $last);
+        // Reset the array to the first element.
+        reset($weekdays);
+        // Retrieve the first week day value.
+        $last = current($weekdays);
+        // Store the corresponding key.
+        $key = key($weekdays);
+        // Remove this week day from the beginning of the array.
+        unset($weekdays[$key]);
+        // Add this week day to the end of the array.
+        $weekdays[$key] = $last;
       }
     }
     return $weekdays;
diff --git a/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php b/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..a88861ca5656378f861f7d5ae96c8cfbd0144f33
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php
@@ -0,0 +1,112 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Datetime\DateHelperTest.
+ */
+
+namespace Drupal\Tests\Core\Datetime;
+
+use Drupal\Core\Datetime\DateHelper;
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Datetime\DateHelper
+ * @group Datetime
+ */
+class DateHelperTest extends UnitTestCase {
+
+  /**
+   * @covers ::weekDaysOrdered
+   * @dataProvider providerTestWeekDaysOrdered
+   */
+  public function testWeekDaysOrdered($first_day, $expected) {
+    $container = new ContainerBuilder();
+    $config = ['system.date' => ['first_day' => $first_day]];
+    $container->set('config.factory', $this->getConfigFactoryStub($config));
+    \Drupal::setContainer($container);
+
+    $weekdays = DateHelper::weekDaysUntranslated();
+    // self::assertSame() MUST be used here as it checks for array key order.
+    $this->assertSame($expected, DateHelper::weekDaysOrdered($weekdays));
+  }
+
+  public function providerTestWeekDaysOrdered() {
+    $data = [];
+    $data[] = [0, [
+      0 => 'Sunday',
+      1 => 'Monday',
+      2 => 'Tuesday',
+      3 => 'Wednesday',
+      4 => 'Thursday',
+      5 => 'Friday',
+      6 => 'Saturday',
+    ]];
+    $data[] = [1, [
+      1 => 'Monday',
+      2 => 'Tuesday',
+      3 => 'Wednesday',
+      4 => 'Thursday',
+      5 => 'Friday',
+      6 => 'Saturday',
+      0 => 'Sunday',
+    ]];
+    $data[] = [2, [
+      2 => 'Tuesday',
+      3 => 'Wednesday',
+      4 => 'Thursday',
+      5 => 'Friday',
+      6 => 'Saturday',
+      0 => 'Sunday',
+      1 => 'Monday',
+    ]];
+    $data[] = [3, [
+      3 => 'Wednesday',
+      4 => 'Thursday',
+      5 => 'Friday',
+      6 => 'Saturday',
+      0 => 'Sunday',
+      1 => 'Monday',
+      2 => 'Tuesday',
+    ]];
+    $data[] = [4, [
+      4 => 'Thursday',
+      5 => 'Friday',
+      6 => 'Saturday',
+      0 => 'Sunday',
+      1 => 'Monday',
+      2 => 'Tuesday',
+      3 => 'Wednesday',
+    ]];
+    $data[] = [5, [
+      5 => 'Friday',
+      6 => 'Saturday',
+      0 => 'Sunday',
+      1 => 'Monday',
+      2 => 'Tuesday',
+      3 => 'Wednesday',
+      4 => 'Thursday',
+    ]];
+    $data[] = [6, [
+      6 => 'Saturday',
+      0 => 'Sunday',
+      1 => 'Monday',
+      2 => 'Tuesday',
+      3 => 'Wednesday',
+      4 => 'Thursday',
+      5 => 'Friday',
+    ]];
+    $data[] = [7, [
+      0 => 'Sunday',
+      1 => 'Monday',
+      2 => 'Tuesday',
+      3 => 'Wednesday',
+      4 => 'Thursday',
+      5 => 'Friday',
+      6 => 'Saturday',
+    ]];
+    return $data;
+  }
+
+}