Fix constraints for non-required fields.

core/modules/datetime_range/src/Plugin/Field/FieldType/DateRangeItem.php

@@ -165,19 +165,29 @@ public function getConstraints() {
       ->getValidationConstraintManager();
     $constraints = parent::getConstraints();
 
-    if ($this->getSetting('optional_values') == static::OPTIONAL_END) {
-      return $constraints;
-    }
-
-    $label = $this->getFieldDefinition()->getLabel();
-    $constraints[] = $constraint_manager
-      ->create('ComplexData', [
-        'end_value' => [
-          'NotNull' => [
-            'message' => $this->t('The @title end date is required', ['@title' => $label]),
+    if (!$this->getFieldDefinition()->isRequired()) {
+      $label = $this->getFieldDefinition()->getLabel();
+      // If the end date triggers constraint validation then test the start date.
+      $constraints[] = $constraint_manager
+        ->create('ComplexData', [
+          'value' => [
+            'NotNull' => [
+              'message' => $this->t('The @title start date is required', ['@title' => $label]),
+            ],
           ],
-        ],
-      ]);
+        ]);
+      // Testing the end date is only needed if not required and not optional.
+      if ($this->getSetting('optional_values') == static::OPTIONAL_NONE) {
+        $constraints[] = $constraint_manager
+          ->create('ComplexData', [
+            'end_value' => [
+              'NotNull' => [
+                'message' => $this->t('The @title end date is required', ['@title' => $label]),
+              ],
+            ],
+          ]);
+      }
+    }
 
     return $constraints;
   }