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
@@ -453,10 +453,13 @@
    *   The number of days in the month.
    */
   public static function daysInMonth($date = NULL) {
-    if ($date && !$date instanceof DrupalDateTime) {
+    if (!isset($date) || empty($date)) {
+      $date = new DrupalDateTime();
+    }
+    elseif (!$date instanceof DrupalDateTime) {
       $date = new DrupalDateTime($date);
     }
-    if ($date && !$date->hasErrors()) {
+    if (!$date->hasErrors()) {
       return $date->format('t');
     }
     return NULL;
@@ -473,10 +476,13 @@
    *   The number of days in the year.
    */
   public static function daysInYear($date = NULL) {
-    if ($date && !$date instanceof DrupalDateTime) {
+    if (!isset($date) || empty($date)) {
+      $date = new DrupalDateTime();
+    }
+    elseif (!$date instanceof DrupalDateTime) {
       $date = new DrupalDateTime($date);
     }
-    if ($date && !$date->hasErrors()) {
+    if (!$date->hasErrors()) {
       if ($date->format('L')) {
         return 366;
       }
@@ -498,10 +504,13 @@
    *   The number of the day in the week.
    */
   public static function dayOfWeek($date = NULL) {
-    if ($date && !$date instanceof DrupalDateTime) {
+    if (!isset($date) || empty($date)) {
+      $date = new DrupalDateTime();
+    }
+    elseif (!$date instanceof DrupalDateTime) {
       $date = new DrupalDateTime($date);
     }
-    if ($date && !$date->hasErrors()) {
+    if (!$date->hasErrors()) {
       return $date->format('w');
     }
     return NULL;
@@ -521,10 +530,13 @@
    *   The name of the day in the week for that date.
    */
   public static function dayOfWeekName($date = NULL, $abbr = TRUE) {
-    if ($date && !$date instanceof DrupalDateTime) {
+    if (!isset($date) || empty($date)) {
+      $date = new DrupalDateTime();
+    }
+    elseif (!$date instanceof DrupalDateTime) {
       $date = new DrupalDateTime($date);
     }
-    if ($date && !$date->hasErrors()) {
+    if (!$date->hasErrors()) {
       $dow = self::dayOfWeek($date);
       $days = $abbr ? self::weekDaysAbbr() : self::weekDays();
       return $days[$dow]->getUntranslatedString();
diff -u b/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php b/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php
--- b/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php
+++ b/core/tests/Drupal/Tests/Core/Datetime/DateHelperTest.php
@@ -164,7 +164,7 @@
    */
   public function testDaysInMonth() {
     // Pass nothing and expect to get NULL.
-    $this->assertNull(DateHelper::daysInMonth());
+    $this->assertEquals(date('t'), DateHelper::daysInMonth());
 
     // December 31st 2022 is a Saturday.
     $value = '2022-12-31 00:00:00';
@@ -182,7 +182,7 @@
    */
   public function testDaysInYear() {
     // Pass nothing and expect to get NULL.
-    $this->assertNull(DateHelper::daysInYear());
+    $this->assertContains(DateHelper::daysInYear(), [365, 366]);
 
     // December 31st 2022 is a Saturday.
     $value = '2022-12-31 00:00:00';
@@ -200,7 +200,7 @@
    */
   public function testDayOfWeek() {
     // Pass nothing and expect to get NULL.
-    $this->assertNull(DateHelper::dayOfWeek());
+    $this->assertContains(DateHelper::dayOfWeek(), range(1, 7));
 
     // December 31st 2022 is a Saturday.
     $value = '2022-12-31 00:00:00';
@@ -218,7 +218,8 @@
    */
   public function testDayOfWeekName() {
     // Pass nothing and expect to get NULL.
-    $this->assertNull(DateHelper::dayOfWeekName());
+    $days_of_week = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'];
+    $this->assertContains(DateHelper::dayOfWeekName(), $days_of_week);
 
     // December 31st 2022 is a Saturday.
     $value = '2022-12-31 00:00:00';