diff -u b/core/lib/Drupal/Core/Datetime/DateHelper.php b/core/lib/Drupal/Core/Datetime/DateHelper.php
--- b/core/lib/Drupal/Core/Datetime/DateHelper.php
+++ b/core/lib/Drupal/Core/Datetime/DateHelper.php
@@ -2,6 +2,8 @@
 
 namespace Drupal\Core\Datetime;
 
+use Drupal\Core\TypedData\Type\StringInterface;
+
 /**
  * Defines Gregorian Calendar date values.
  *
@@ -453,10 +455,10 @@
    *   The number of days in the month.
    */
   public static function daysInMonth($date = NULL) {
-    if (!$date instanceof DrupalDateTime) {
+    if ($date && !$date instanceof DrupalDateTime) {
       $date = new DrupalDateTime($date);
     }
-    if (!$date->hasErrors()) {
+    if ($date && !$date->hasErrors()) {
       return $date->format('t');
     }
     return NULL;
@@ -473,10 +475,10 @@
    *   The number of days in the year.
    */
   public static function daysInYear($date = NULL) {
-    if (!$date instanceof DrupalDateTime) {
-      $date = new DrupalDateTime();
+    if ($date && !$date instanceof DrupalDateTime) {
+      $date = new DrupalDateTime($date);
     }
-    if (!$date->hasErrors()) {
+    if ($date && !$date->hasErrors()) {
       if ($date->format('L')) {
         return 366;
       }
@@ -498,10 +500,10 @@
    *   The number of the day in the week.
    */
   public static function dayOfWeek($date = NULL) {
-    if (!$date instanceof DrupalDateTime) {
+    if ($date && !$date instanceof DrupalDateTime) {
       $date = new DrupalDateTime($date);
     }
-    if (!$date->hasErrors()) {
+    if ($date && !$date->hasErrors()) {
       return $date->format('w');
     }
     return NULL;
@@ -521,12 +523,15 @@
    *   The name of the day in the week for that date.
    */
   public static function dayOfWeekName($date = NULL, $abbr = TRUE) {
-    if (!$date instanceof DrupalDateTime) {
+    if ($date && !$date instanceof DrupalDateTime) {
       $date = new DrupalDateTime($date);
     }
-    $dow = self::dayOfWeek($date);
-    $days = $abbr ? self::weekDaysAbbr() : self::weekDays();
-    return $days[$dow];
+    if ($date && !$date->hasErrors()) {
+      $dow = self::dayOfWeek($date);
+      $days = $abbr ? self::weekDaysAbbr() : self::weekDays();
+      return $days[$dow];
+    }
+    return NULL;
   }
 
 }
only in patch2:
unchanged:
--- a/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php
+++ b/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php
@@ -4,6 +4,7 @@
 
 use Drupal\Core\Datetime\DateHelper;
 use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Core\Language\Language;
 use Drupal\Tests\UnitTestCase;
 
 /**
@@ -12,6 +13,36 @@
  */
 class DateHelperTest extends UnitTestCase {
 
+  /**
+   * The language manager.
+   *
+   * @var \Drupal\Core\Language\LanguageManagerInterface|\PHPUnit\Framework\MockObject\MockObject
+   */
+  protected $languageManager;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $container = new ContainerBuilder();
+    $config = ['system.date' => ['first_day' => 'Sunday']];
+    $container->set('config.factory', $this->getConfigFactoryStub($config));
+
+    $this->languageManager = $this->createMock('\Drupal\Core\Language\LanguageManagerInterface');
+    $language = new Language(['langcode' => 'en']);
+    $this->languageManager->expects($this->any())
+      ->method('getDefaultLanguage')
+      ->will($this->returnValue($language));
+    $this->languageManager->expects($this->any())
+      ->method('getCurrentLanguage')
+      ->will($this->returnValue($language));
+    $container->set('language_manager', $this->languageManager);
+
+    \Drupal::setContainer($container);
+  }
+
   /**
    * @covers ::weekDaysOrdered
    * @dataProvider providerTestWeekDaysOrdered
@@ -128,4 +159,78 @@ public function providerTestWeekDaysOrdered() {
     return $data;
   }
 
+  /**
+   * @covers ::daysInMonth
+   */
+  public function testDaysInMonth() {
+    // Pass nothing and expect to get NULL.
+    $this->assertNull(DateHelper::daysInMonth());
+
+    // December 31st 2022 is a Saturday.
+    $value = '2022-12-31 00:00:00';
+    $dateString = DateHelper::daysInMonth($value);
+    $this->assertEquals('31', $dateString);
+
+    // November 30th 2020 is a Monday.
+    $value = '2020-11-30 00:00:00';
+    $dateString = DateHelper::daysInMonth($value);
+    $this->assertEquals('30', $dateString);
+  }
+
+  /**
+   * @covers ::daysInYear
+   */
+  public function testDaysInYear() {
+    // Pass nothing and expect to get NULL.
+    $this->assertNull(DateHelper::daysInYear());
+
+    // December 31st 2022 is a Saturday.
+    $value = '2022-12-31 00:00:00';
+    $dateString = DateHelper::daysInYear($value);
+    $this->assertEquals('365', $dateString);
+
+    // November 30th 2020 is a Monday. 2020 is a leap year.
+    $value = '2020-11-30 00:00:00';
+    $dateString = DateHelper::daysInYear($value);
+    $this->assertEquals('366', $dateString);
+  }
+
+
+  /**
+   * @covers ::dayOfWeek
+   */
+  public function testDayOfWeek() {
+    // Pass nothing and expect to get NULL.
+    $this->assertNull(DateHelper::dayOfWeek());
+
+    // December 31st 2022 is a Saturday.
+    $value = '2022-12-31 00:00:00';
+    $dateString = DateHelper::dayOfWeek($value);
+    $this->assertEquals('6', $dateString);
+
+    // November 30th 2020 is a Monday.
+    $value = '2020-11-30 00:00:00';
+    $dateString = DateHelper::dayOfWeek($value);
+    $this->assertEquals('1', $dateString);
+  }
+
+
+  /**
+   * @covers ::dayOfWeekName
+   */
+  public function testDayOfWeekName() {
+    // Pass nothing and expect to get NULL.
+    $this->assertNull(DateHelper::dayOfWeekName());
+
+    // December 31st 2022 is a Saturday.
+    $value = '2022-12-31 00:00:00';
+    $dateString = DateHelper::dayOfWeekName($value);
+    $this->assertEquals('Sat', $dateString->getUntranslatedString());
+
+    // November 30th 2020 is a Monday.
+    $value = '2020-11-30 00:00:00';
+    $dateString = DateHelper::dayOfWeekName($value);
+    $this->assertEquals('Mon', $dateString->getUntranslatedString());
+  }
+
 }