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
@@ -454,14 +454,8 @@
    */
   public static function daysInMonth($date = NULL) {
     if (!empty($date)) {
-      if (!$date instanceof DrupalDateTime) {
-        $date = new DrupalDateTime($date);
-      }
-      if (!$date->hasErrors()) {
-        return $date->format('t');
-      }
+      return NULL;
     }
-    return NULL;
   }
 
   /**